Delete a child account

You can transfer the account.

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 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      

One way to do this would be to log into the unwanted "child" account and request deletion of the Apple ID. 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 the referenced links below.

Family Sharing and Apple ID for your child - https://support.apple.com/HT201084

- "Your email address is the rescue email address for your child’s account, and you can use it with the security questions you provided to reset a forgotten password. You can keep your your child's account more secure with two-factor authentication."

- "If you need to see the options you have for managing your child's account, you can sign in with their Apple ID and password at https://privacy.apple.com/account."

How to delete your Apple ID account - https://support.apple.com/HT208504

- Can I delete my child’s account?

"Your child can also request deletion of their Apple ID, which requires your approval."

You could also contact Apple about this request: Click here --> https://support.apple.com/choose-country-region/media-services 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.