If you have gamepass ultimate you have a game license for both pc and xbox, however you can not play together if the primary person on the account is the Xbox user, my child's account only works on the xbox and not his windows 10 pc. This means we can not play together on network play despite forking over a hefty subscription fee every month.

The only way I can "fix this" is to cancel game pass ultimate, sign him up as an adult (exposing him to things he shouldn't see) and separately sign him up to game pass for PC.

This needs some attention, it's supposed to be a family game but you can't play with your family.