Hey Stacy618,
If we understand correctly, your child has two Apple ID's and you would like to delete one of them and add the other to your Family Sharing account. If that's correct, keep in mind that any purchase made with that Apple ID will be lost as purchases are linked to the Apple ID used and cannot be transferred.
If the account shows as a child account, the steps to delete the account are a bit different. Check the section "Can I delete my child’s account?" for information. Otherwise, the steps to delete the account can be found here: How to delete your Apple ID account - Apple Support
These guides will help with setting up Family Sharing and setting restrictions:
Set up Family Sharing - Apple Support
Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support
We hope this helps.
Take care!