Migrating to a new Apple ID? Be prepared to lose data for your games
The Apple ID is the center of the Apple universe, particularly for purchases and iCloud synchronization. That becomes a tricky issue whenever a single person's Apple ID is used by multiple people, whether spouses, partners, or roommates, or—as is quite common—by children before a parent grants them their own Apple ID.
It's easy enough to create an Apple ID for a child. If they're under 13, you have to have Family Sharing enabled and you create the account as part of that system. From 13 on up, you can still use Family Sharing, or you and they can create a freestanding account. (That cutoff age is different in some countries.)