Move data from parent account to child account
Purchases, activity, and app/game progress are tied to an Apple ID.  In this case, they're tied to your Apple ID and there's no way to transfer this to a different Apple ID (your child's ID).

Some apps allow family sharing and some don't.  Because you've enabled that feature, your child will be able to access for free any apps you've already purchased.  In-app purchases will not be accessible by your child.  

The activity/progress made on those apps under your ID will not be transferred to your child.  As of today, there's no way to transfer that that info.

Your child's account will be setup as a brand new account.  See Apple's Family Sharing site for more info.