Family Share, Game Center: Create Child Apple ID

As Arcade games seem to often only offer 2- or multi-player via "co-play" with an invite, but we have two kids trying to play peacefully and cooperatively on the same Apple TV screen, I tried to set up under Family Share a Child Apple ID. I'd go through all the steps, verify her email, etc. and then get an error message window "cannot be created at this time." This happened multiple times. How is this so excruciating unusable? "Invite to Co-Play" doesn't find anything when it searches our WiFi despite both game controllers being paired and on and in use.

What can we do to simply have our two children able to play TOGETHER in Arcade on the same screen? Are all the two-player games using "co-play" not really able to do it without being on separate devices?