Child account has settings enabled to join multi-player games with friends only, in Xbox Live.
Parent account has Realms subscription to enable child + child's friends to play in safe environment.
Child account cannot join realm from any device. XBox support already contacted and have ruled out problem with account- all settings correct. Mojang support already contacted and very unhelpful, pointing me here to raise a bug on their behalf (very strange development practice, frankly).
Child account can join server games, can invite friends to his own individual worlds, and can join friends' worlds too (alongside unknown friends-of-friends).

"Shut up and take my money"- I want to pay you a monthly subscription for realms to work but at the moment it is completely pointless until clients and realms work properly with child accounts.