diff --git a/apps/backend/src/app/modules/form/admin-form/__tests__/admin-form.service.spec.ts b/apps/backend/src/app/modules/form/admin-form/__tests__/admin-form.service.spec.ts
index fbd5464a53..100191f454 100644
--- a/apps/backend/src/app/modules/form/admin-form/__tests__/admin-form.service.spec.ts
+++ b/apps/backend/src/app/modules/form/admin-form/__tests__/admin-form.service.spec.ts
@@ -2190,6 +2190,44 @@ describe('admin-form.service', () => {
       )
     })
 
+    it('defaults to v2 and strips disallowed options when updating a children field in a Multi-respondent form', async () => {
+      // Arrange
+      const fieldToUpdate = generateDefaultField(BasicField.Children)
+      const mockNewField = {
+        ...fieldToUpdate,
+        version: undefined,
+        allowMultiple: true,
+        childrenSubFields: [
+          MyInfoChildAttributes.ChildName,
+          MyInfoChildAttributes.ChildSecondaryRace,
+        ],
+      } as unknown as FieldUpdateDto
+      const mockForm = {
+        form_fields: [fieldToUpdate],
+        responseMode: FormResponseMode.Multirespondent,
+        updateFormFieldById: jest
+          .fn()
+          .mockResolvedValue({ form_fields: [mockNewField] }),
+      } as unknown as IPopulatedForm
+
+      // Act
+      await AdminFormService.updateFormField(
+        mockForm,
+        fieldToUpdate._id,
+        mockNewField,
+      )
+
+      // Assert
+      expect(mockForm.updateFormFieldById).toHaveBeenCalledWith(
+        fieldToUpdate._id,
+        expect.objectContaining({
+          version: ChildrenFieldVersion.V2,
+          allowMultiple: false,
+          childrenSubFields: [MyInfoChildAttributes.ChildName],
+        }),
+      )
+    })
+
     it('leaves a legacy children field untouched on update', async () => {
       // Arrange
       const fieldToUpdate = generateDefaultField(BasicField.Children)
@@ -2387,13 +2425,25 @@ describe('admin-form.service', () => {
       )
     })
 
+    it('defaults to version 2 for a Multi-respondent form even when children-v2 is disabled', async () => {
+      const insertFormField = await createChildrenFieldWith({
+        childrenV2Enabled: false,
+        responseMode: FormResponseMode.Multirespondent,
+      })
+
+      expect(insertFormField).toHaveBeenCalledWith(
+        expect.objectContaining({ version: ChildrenFieldVersion.V2 }),
+        undefined,
+      )
+    })
+
     it('strips Secondary Race and Allow-Multiple when stamping v2', async () => {
       const insertFormField = jest.fn().mockResolvedValue({
         form_fields: [generateDefaultField(BasicField.Children)],
       })
       const mockForm = {
         form_fields: [],
-        responseMode: FormResponseMode.Encrypt,
+        responseMode: FormResponseMode.Multirespondent,
         insertFormField,
       } as unknown as IPopulatedForm
       const formCreateParams = {
@@ -2411,7 +2461,7 @@ describe('admin-form.service', () => {
         mockForm,
         formCreateParams,
         undefined,
-        { childrenV2Enabled: true },
+        { childrenV2Enabled: false },
       )
 
       expect(insertFormField).toHaveBeenCalledWith(
diff --git a/apps/backend/src/app/modules/form/admin-form/admin-form.service.ts b/apps/backend/src/app/modules/form/admin-form/admin-form.service.ts
index b40ecc27ad..985c98e138 100644
--- a/apps/backend/src/app/modules/form/admin-form/admin-form.service.ts
+++ b/apps/backend/src/app/modules/form/admin-form/admin-form.service.ts
@@ -923,13 +923,21 @@ export const updateFormField = (
   const _newField = insertTableShortTextColumnDefaultValidationOptions(newField)
 
   // children-v2 (ADR-0001): the v2 invariants must hold on edits too, not only
-  // on create. If the payload is stamped v2, strip Secondary Race /
-  // Allow-Multiple so a crafted update payload can't reintroduce them. We never
-  // downgrade an existing version here.
+  // on create. A children field is v2 if the form is Multi-respondent (where v2
+  // is the default) or the payload is already stamped v2; in that case strip
+  // Secondary Race / Allow-Multiple so a crafted update payload can't
+  // reintroduce them. Note the v2 test differs from createFormField on purpose:
+  // create ignores any payload `version` (the flag/MRF is the authoritative
+  // gate, so a crafted create can't opt in), whereas update trusts the
+  // already-stamped `version` and never re-derives v2-ness from the flag. We
+  // never downgrade an existing version here.
   if (_newField.fieldType === BasicField.Children) {
     const childrenField = _newField as ChildrenCompoundFieldBase
-    const isV2 = childrenField.version === ChildrenFieldVersion.V2
+    const isV2 =
+      form.responseMode === FormResponseMode.Multirespondent ||
+      childrenField.version === ChildrenFieldVersion.V2
     if (isV2) {
+      childrenField.version = ChildrenFieldVersion.V2
       enforceChildrenV2Invariants(childrenField)
     }
   }
@@ -1014,7 +1022,7 @@ export const duplicateFormField = (
 }
 
 /**
- * Enforces the children-v2 data invariants (ADR-0001/0002) on a children field
+ * Enforces the children-v2 data invariants (ADR-0001) on a children field
  * before it is written: v2 has neither Secondary Race nor Allow-Multiple, so we
  * drop them from the field data — not just from the builder UI — so the
  * invariant holds however the field became v2 (fresh create, MRF default, or
@@ -1054,11 +1062,14 @@ export const createFormField = (
   }
 
   // Children version is stamped server-side (ADR-0001), so the gate is
-  // authoritative — a hand-crafted payload can't opt in. In Storage/Encrypt
-  // mode v2 is whitelist-gated by the children-v2 flag.
+  // authoritative — a hand-crafted payload can't opt in. v2 is the **default
+  // in Multi-respondent forms** (the definitive children field lives in unified
+  // modes); in Storage/Encrypt mode it stays whitelist-gated by children-v2.
   if (newField.fieldType === BasicField.Children) {
     const childrenField = newField as ChildrenCompoundFieldBase
-    const isV2 = !!opts?.childrenV2Enabled
+    const isMultirespondent =
+      form.responseMode === FormResponseMode.Multirespondent
+    const isV2 = isMultirespondent || !!opts?.childrenV2Enabled
     childrenField.version = isV2
       ? ChildrenFieldVersion.V2
       : ChildrenFieldVersion.Legacy
diff --git a/apps/frontend/src/features/admin-form/create/builder-and-design/BuilderAndDesignDrawer/EditFieldDrawer/edit-fieldtype/EditMyInfoChildren/EditMyInfoChildren.tsx b/apps/frontend/src/features/admin-form/create/builder-and-design/BuilderAndDesignDrawer/EditFieldDrawer/edit-fieldtype/EditMyInfoChildren/EditMyInfoChildren.tsx
index 7fb027a351..75b68de5d7 100644
--- a/apps/frontend/src/features/admin-form/create/builder-and-design/BuilderAndDesignDrawer/EditFieldDrawer/edit-fieldtype/EditMyInfoChildren/EditMyInfoChildren.tsx
+++ b/apps/frontend/src/features/admin-form/create/builder-and-design/BuilderAndDesignDrawer/EditFieldDrawer/edit-fieldtype/EditMyInfoChildren/EditMyInfoChildren.tsx
@@ -3,7 +3,7 @@ import { BiCheck, BiData, BiX } from 'react-icons/bi'
 import { Box, FormControl, HStack, Icon, Text, VStack } from '@chakra-ui/react'
 import { extend } from 'lodash'
 
-import { isChildrenV2Field, MyInfoChildAttributes } from 'formsg-shared/types'
+import { MyInfoChildAttributes } from 'formsg-shared/types'
 
 import { SINGPASS_FAQ } from '~constants/links'
 import { MultiSelect } from '~components/Dropdown'
@@ -12,6 +12,8 @@ import Link from '~components/Link'
 import { Toggle } from '~components/Toggle/Toggle'
 
 import { CREATE_MYINFO_CHILDREN_SUBFIELDS_OPTIONS } from '~features/admin-form/create/builder-and-design/constants'
+import { useCreateTabForm } from '~features/admin-form/create/builder-and-design/useCreateTabForm'
+import { isChildrenV2InBuilder } from '~features/myinfo/utils'
 
 import { CreatePageDrawerContentContainer } from '../../../../../common'
 import { FormFieldDrawerActions } from '../common/FormFieldDrawerActions'
@@ -49,9 +51,12 @@ export const EditMyInfoChildren = ({
   field,
 }: EditMyInfoChildrenProps): JSX.Element => {
   const extendedField = extendWithMyInfo(field)
-  // children-v2 (ADR-0001): drop Secondary Race + Allow-Multiple and show the
-  // new description when the field is stamped v2.
-  const isV2 = isChildrenV2Field(field)
+  const { data: form } = useCreateTabForm()
+  // children-v2 (ADR-0001): drop Secondary Race + Allow-Multiple and show
+  // the new description. v2 applies when the field is stamped v2 OR the form is
+  // Multi-respondent (v2 is the default there, stamped on save) — so the editor
+  // reflects v2 immediately, not only after the field is persisted.
+  const isV2 = isChildrenV2InBuilder(field, form?.responseMode)
   const subFieldOptions = isV2
     ? CREATE_MYINFO_CHILDREN_SUBFIELDS_OPTIONS.filter(
         (option) => option.value !== MyInfoChildAttributes.ChildSecondaryRace,
diff --git a/apps/frontend/src/features/myinfo/utils/canAddChildrenField.test.ts b/apps/frontend/src/features/myinfo/utils/canAddChildrenField.test.ts
index 82ce5b0893..be355baea8 100644
--- a/apps/frontend/src/features/myinfo/utils/canAddChildrenField.test.ts
+++ b/apps/frontend/src/features/myinfo/utils/canAddChildrenField.test.ts
@@ -14,19 +14,19 @@ describe('canAddChildrenField', () => {
     ).toBe(true)
   })
 
-  it('hides children without the beta flag', () => {
+  it('allows children in Multi-respondent mode with the beta flag (v2 is the default there)', () => {
     expect(
       canAddChildrenField({
-        hasChildrenBetaFlag: false,
-        responseMode: FormResponseMode.Encrypt,
+        hasChildrenBetaFlag: true,
+        responseMode: FormResponseMode.Multirespondent,
       }),
-    ).toBe(false)
+    ).toBe(true)
   })
 
-  it('hides children in Multi-respondent mode (arrives with MRF support)', () => {
+  it('hides children without the beta flag, regardless of mode', () => {
     expect(
       canAddChildrenField({
-        hasChildrenBetaFlag: true,
+        hasChildrenBetaFlag: false,
         responseMode: FormResponseMode.Multirespondent,
       }),
     ).toBe(false)
diff --git a/apps/frontend/src/features/myinfo/utils/canAddChildrenField.ts b/apps/frontend/src/features/myinfo/utils/canAddChildrenField.ts
index 91f641c846..692e4547f6 100644
--- a/apps/frontend/src/features/myinfo/utils/canAddChildrenField.ts
+++ b/apps/frontend/src/features/myinfo/utils/canAddChildrenField.ts
@@ -6,6 +6,9 @@ import { FormResponseMode } from 'formsg-shared/types'
  * - **Encrypt (Storage)** mode: available with the children beta flag (today's
  *   behaviour). v1 vs v2 there is governed separately by the `children-v2`
  *   GrowthBook flag at create time.
+ * - **Multi-respondent** mode: available with the children beta flag —
+ *   children-v2 is the **default** here (the definitive children field lives in
+ *   the unified-modes / answerObject-v4 world), so no extra flag is needed.
  * - All other modes (e.g. Email): not supported.
  */
 export const canAddChildrenField = ({
@@ -18,5 +21,8 @@ export const canAddChildrenField = ({
   if (!hasChildrenBetaFlag) {
     return false
   }
-  return responseMode === FormResponseMode.Encrypt
+  return (
+    responseMode === FormResponseMode.Encrypt ||
+    responseMode === FormResponseMode.Multirespondent
+  )
 }
diff --git a/apps/frontend/src/features/myinfo/utils/index.ts b/apps/frontend/src/features/myinfo/utils/index.ts
index c9def73649..18cc3cc359 100644
--- a/apps/frontend/src/features/myinfo/utils/index.ts
+++ b/apps/frontend/src/features/myinfo/utils/index.ts
@@ -3,4 +3,5 @@ export * from './augmentWithMyInfoDisplayValue'
 export * from './canAddChildrenField'
 export * from './extractPreviewValue'
 export * from './hasExistingFieldValue'
+export * from './isChildrenV2InBuilder'
 export * from './isMyInfo'
diff --git a/apps/frontend/src/features/myinfo/utils/isChildrenV2InBuilder.test.ts b/apps/frontend/src/features/myinfo/utils/isChildrenV2InBuilder.test.ts
new file mode 100644
index 0000000000..5a4e60da7c
--- /dev/null
+++ b/apps/frontend/src/features/myinfo/utils/isChildrenV2InBuilder.test.ts
@@ -0,0 +1,47 @@
+import { describe, expect, it } from 'vitest'
+
+import {
+  BasicField,
+  ChildrenFieldVersion,
+  FormResponseMode,
+} from 'formsg-shared/types'
+
+import { isChildrenV2InBuilder } from './isChildrenV2InBuilder'
+
+describe('isChildrenV2InBuilder', () => {
+  it('is true for a children field already stamped v2 (Encrypt)', () => {
+    expect(
+      isChildrenV2InBuilder(
+        { fieldType: BasicField.Children, version: ChildrenFieldVersion.V2 },
+        FormResponseMode.Encrypt,
+      ),
+    ).toBe(true)
+  })
+
+  it('is true for a children field on a Multi-respondent form even before it is stamped', () => {
+    expect(
+      isChildrenV2InBuilder(
+        { fieldType: BasicField.Children },
+        FormResponseMode.Multirespondent,
+      ),
+    ).toBe(true)
+  })
+
+  it('is false for an unstamped children field on Encrypt', () => {
+    expect(
+      isChildrenV2InBuilder(
+        { fieldType: BasicField.Children },
+        FormResponseMode.Encrypt,
+      ),
+    ).toBe(false)
+  })
+
+  it('is false for a non-children field, even on Multi-respondent', () => {
+    expect(
+      isChildrenV2InBuilder(
+        { fieldType: BasicField.ShortText },
+        FormResponseMode.Multirespondent,
+      ),
+    ).toBe(false)
+  })
+})
diff --git a/apps/frontend/src/features/myinfo/utils/isChildrenV2InBuilder.ts b/apps/frontend/src/features/myinfo/utils/isChildrenV2InBuilder.ts
new file mode 100644
index 0000000000..5c74173e0c
--- /dev/null
+++ b/apps/frontend/src/features/myinfo/utils/isChildrenV2InBuilder.ts
@@ -0,0 +1,28 @@
+import {
+  BasicField,
+  ChildrenFieldVersion,
+  FormResponseMode,
+} from 'formsg-shared/types'
+
+/**
+ * Whether a children field should behave as v2 in the builder UI for a form of
+ * the given response mode.
+ *
+ * Mirrors the server-side rule in `createFormField`: a children field is v2 if
+ * it is already stamped `version: 2`, **or** the form is Multi-respondent
+ * (where v2 is the default). The mode check matters because an MRF children
+ * field is only stamped on save — without it the editor would briefly show the
+ * v1 options (Secondary Race, Allow-Multiple) for a field that is really v2.
+ */
+export const isChildrenV2InBuilder = (
+  field: { fieldType: BasicField; version?: ChildrenFieldVersion },
+  responseMode?: FormResponseMode,
+): boolean => {
+  if (field.fieldType !== BasicField.Children) {
+    return false
+  }
+  return (
+    field.version === ChildrenFieldVersion.V2 ||
+    responseMode === FormResponseMode.Multirespondent
+  )
+}
diff --git a/packages/sdk/spec/adaptor/adapt-v3-to-v4.spec.ts b/packages/sdk/spec/adaptor/adapt-v3-to-v4.spec.ts
index 8555bd3cf3..63fca27f6f 100644
--- a/packages/sdk/spec/adaptor/adapt-v3-to-v4.spec.ts
+++ b/packages/sdk/spec/adaptor/adapt-v3-to-v4.spec.ts
@@ -298,6 +298,79 @@ describe('adaptV3ToV4', () => {
         },
       })
     })
+
+    // Characterisation test for the children-v2 storage shape (ADR-0001).
+    // The v4 answerObject already produces exactly what v2 needs for a local
+    // child: a single un-numbered `[Myinfo] Children` field whose answer nests
+    // each sub-field under one child key — no `Child 1 / Child 2` numbering in
+    // the question label or the sub-field keys. This locks that shape so it is
+    // ready to flip on when storage-mode v4 lands; the per-child `type`
+    // (nuclear/sponsored) stamp is added with sponsored support (slice 03).
+    it('produces the children-v2 shape: one un-numbered local child, nested per sub-field', () => {
+      const v3: FormFieldsV3 = {
+        childrenField: {
+          fieldType: 'children',
+          answer: {
+            child: [['Tan Wen Jie', 'S1234567A', 'MALE']],
+            childFields: ['childname', 'childbirthcertno', 'childgender'],
+          },
+        },
+      }
+
+      const result = adaptV3ToV4(v3, {
+        formFields: {
+          childrenField: {
+            question: 'Children',
+            myInfo: { attr: 'childrenbirthrecords' },
+          },
+        },
+      })
+
+      expect(result.childrenField.question).toBe('[Myinfo] Children')
+      expect(result.childrenField.question).not.toMatch(/Child\s*\d/)
+      expect(Object.keys(result.childrenField.answer)).toEqual(['child0'])
+      expect(result.childrenField.answer).toEqual({
+        child0: {
+          value: {
+            childname: {
+              value: 'Tan Wen Jie',
+              myInfo: { attr: 'childname' },
+            },
+            childbirthcertno: {
+              value: 'S1234567A',
+              myInfo: { attr: 'childbirthcertno' },
+            },
+            childgender: { value: 'MALE', myInfo: { attr: 'childgender' } },
+          },
+        },
+      })
+    })
+
+    // children-v2 sponsored support (ADR-0002): a per-child record type
+    // (nuclear/sponsored) is threaded into ChildEntryV4.type when the V3
+    // answer carries a parallel `childrenTypes` array.
+    it('stamps per-child record type from childrenTypes', () => {
+      const v3: FormFieldsV3 = {
+        field1: {
+          fieldType: 'children',
+          answer: {
+            child: [
+              ['Local Tan', 'S1111111A'],
+              ['Sponsored Lee', 'T2222222B'],
+            ],
+            childFields: ['childname', 'childbirthcertno'],
+            childrenTypes: ['nuclear', 'sponsored'],
+          },
+        },
+      }
+
+      const result = adaptV3ToV4(v3)
+
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const answer = result.field1.answer as any
+      expect(answer.child0.type).toBe('nuclear')
+      expect(answer.child1.type).toBe('sponsored')
+    })
   })
 
   describe('address field', () => {
diff --git a/packages/sdk/src/adapt-v3-to-v4.ts b/packages/sdk/src/adapt-v3-to-v4.ts
index ec77f51b1d..b256cacf78 100644
--- a/packages/sdk/src/adapt-v3-to-v4.ts
+++ b/packages/sdk/src/adapt-v3-to-v4.ts
@@ -85,6 +85,8 @@ const convertTableAnswer = (answer: Record<string, string>[]): TableAnswerV4 =>
 const convertChildrenAnswer = (answer: {
   child: string[][]
   childFields: string[]
+  // children-v2: per-child record type (nuclear/sponsored), parallel to `child`.
+  childrenTypes?: string[]
 }): ChildrenAnswerV4 => {
   const result: ChildrenAnswerV4 = {}
   for (let i = 0; i < answer.child.length; i++) {
@@ -98,7 +100,8 @@ const convertChildrenAnswer = (answer: {
         myInfo: { attr },
       }
     }
-    result[childKey] = { value }
+    const type = answer.childrenTypes?.[i]
+    result[childKey] = { value, ...(type !== undefined && { type }) }
   }
   return result
 }
diff --git a/packages/sdk/src/adapt-v4-to-v3.ts b/packages/sdk/src/adapt-v4-to-v3.ts
index c412b2202e..25c79ba90f 100644
--- a/packages/sdk/src/adapt-v4-to-v3.ts
+++ b/packages/sdk/src/adapt-v4-to-v3.ts
@@ -1,4 +1,7 @@
-import { ADDRESS_SUBFIELD_KEYS, GENERIC_STRING_FIELD_TYPES } from './constants-v4'
+import {
+  ADDRESS_SUBFIELD_KEYS,
+  GENERIC_STRING_FIELD_TYPES,
+} from './constants-v4'
 import { FieldType, FormFieldsV3 } from './types'
 import {
   AddressAnswerV4,
@@ -104,6 +107,10 @@ function convertChildrenAnswerToV3(answer: ChildrenAnswerV4): {
     child.push(childValues)
   }
 
+  // children-v2 (ADR-0001): the per-child record `type` that adaptV3ToV4 stamps
+  // onto ChildEntryV4 is intentionally NOT reconstructed here — the v3 answer
+  // shape has no slot for it, and this reverse path is legacy/decrypt-only, so a
+  // v4→v3 conversion deliberately drops `type` rather than round-tripping it.
   return { child, childFields }
 }