How could I take away family sharing from my child device

You need to transfer them to a new family or delete the Apple ID they are using.

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

Some people have been able to delete extra accounts using the steps in the articles below. Make sure you sign out from the places in the 4th link before deleting the ID. If you fail to sign out of your device before deleting the ID, the device may become useless.

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