Remove a child under 13 from family share

Some people have been able to delete the account. Make sure you sign out from the places in the 4th link before deleting the ID.

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      

You can do so only if you move the child to another family first. Then, if you are the organizer, just leave family sharing yourself to disband the family. See Leave Family Sharing - Apple Support