How do you remove a child Apple ID in family sharing

If the child is under 13, you can transfer them to another Family.

Family Sharing -  Transfer a child to a different family group - Apple Support   *

If 13 years old or older, they can remove themselves or the organizer can remove them.

Family Sharing - Leave.  *

Another option is to try to delete their Apple ID. Some people have been able to delete extra accounts using the steps in the articles below.

All purchases made using an Apple ID are tied to the ID. Delete the ID and you will need to download them using your new ID. Once your Apple ID is deleted, you can't use the email address that you used with it to create a new Apple ID. In addition, you can't use that email address with another existing Apple ID except as a rescue email. Also, if your primary Apple ID email address is with a Custom Email Domain, it can never be reused to create an Apple ID after your Apple ID is deleted, even if the domain ownership changes..  Make sure that you sign out of all your devices to avoid issues with apps that use your Apple ID account or iCloud. After your account is deleted, you can't sign out of iCloud or turn off Find My iPhone Activation Lock on your devices. If you forget to sign out, you might not be able to use your device when your account is deleted.

How to delete your Apple ID account.   *  

Apple ID – How to Delete.  -

Delete Apple ID (new).   -

What to do after you change your Apple ID or password - Apple Support   -