How do you delete a child from a family account?

You can't remove a child (age varies by location but often under 13) from Family Sharing. You can either:

Move a child to another group using Family Sharing as outlined in this document: Move a child to another group using Family Sharing - Apple Support

or:

Delete their Apple ID account entirely. Doing this means the child will no longer be able to re-download past purchases.

One way to do this would be to log into the unwanted "child" account and request deletion of the AppleID. You should have the ID and password from when you first set up the account, or can recover the password according to information provided in this support article:

Family Sharing and Apple ID for your child - Create an Apple ID for your child - Apple Support

- "If you are the family organizer or guardian, you can help reset your child's Apple ID password using your iPhone with iOS 16 or later or iPad with iPadOS 16 or later and two-factor authentication turned on for your Apple ID. --> Change your Apple ID password - Apple Support

Next, from the document : How to delete your Apple ID account - How to delete your Apple ID account - Apple Support

- 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."

You could also contact Apple about this request: Click here --> Choose your country or region - Official Apple Support Select your country, then a product and work your way down the menu to the desired issue. If you don't see one then try a different product until you reach a telephone or chat help request and ask to be redirected.