How to delete an account from family sharing

Based on your screen shot, the person is 4 years, so you can't remove them from your account. You can transfer them using the link I posted above.

The other option is to try deleting their account. Some people have posted they could do that. 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      

See the "Remove someone ..." section of Leave Family Sharing - Apple Support

If less than 13 years old.

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