we have ultimate game pass on console but when we tried to use it on a phone for childs...

hello

I got an xbox a month ago and have ultimate game pass on it . It is registered to me. My child just tried to play on a moblie phone logged in as them ( they play as them on the xbox) but no subscription shows up.

How do i register the phone or their account so they can play games on their phone. I have looked online for an answer but could not find one and did not want to change the setting without checking here what to do.

Thank you for your help

:)