Change a child’s birthday

You need to contact Apple if the birthday for a child (usually under 13) is wrong.

See:

Update the birth date associated with your Apple ID - Apple Support

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Change a birth date for a child

To change a birth date to any age below 13,* you must be a member of a Family Sharing group, and your family organizer must approve the change. After you change a birth date to any age below 13, you can't edit it.* Learn more about Family Sharing. 

1. Update the birth date.
2. Follow the onscreen steps to send an email request to your family organizer.
3. Your family organizer will get an email with instructions to approve the request. 

*Age varies by country or region. If you need to change the birth date for a child, contact Apple Support. 

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Use that link or this:

Contact Apple for support and service - Apple Support

to contact Apple Support

Good luck!