Issues with my twins' AppleIDs creating accounts in apps

My twins have their own iPads, their own email addresses, and their own Apple IDs. I have them under my Family Share, with me as the Organizer.

Here's the problem: One child is able to login to Monopoly GO (as an example), and be able to play the game with Sign In with Apple ID. The other child cannot.

The two Apple IDs were not made at the same time, so I'm wondering if Apple changed something recently to restrict that feature until the child turns 13. I don't agree with this policy, if it's something recent. In any case, it shouldn't make a difference if I have both accounts under my Family Share.

Can anyone give me some insight as to what is going on? I've read that other are having this issue, and there's ZERO support from Apple on this.

Thanks, everyone.