Adding my child’s already existing account

Hey there, Siren1921!

Thanks so much for using Apple Support Communities.

You can definitely add a child with an existing Apple ID to your Family Sharing group. Here is how you can do that: Set up Family Sharing. It should take you right there, but if not you can scroll down to the section titled 'Invite people to join your family' for the steps you'll need to invite a new user to your family. It looks like that is an article you may have seen already but it's definitely the best resource for this issue.

We hope this does the trick for you!

Cheers.