Deceased child
Removing a deceased child from family sharing without knowing password for the child’s Apple ID
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For information about how to request access to a deceased family member's Apple accounts, and through this their equipment, click here --> How to request access to a deceased family member's Apple account - Apple Support
Then make a request from the account for deletion: How to delete your Apple ID account - How to delete your Apple ID account - Apple Support
Section: Can I delete my child’s account?
"Your child can also request deletion of their Apple ID, which requires your approval.
The approval request will be sent via email to the primary Apple ID email address of your family organizer account. When approving the request from this email, you will go through the process of deleting the child account."