I set up a Child Xbox Live account for my son so he could play Minecraft on his iPad.

I also have Minecraft on my iPad, and we have successfully played together many times in a LAN mode using local worlds saved on each other’s iPad.

I signed up for Realm 30-day trial so we could play in a world that is not tied to either iPad (and works from anywhere).

I changed the two settings (create & join clubs and join multiplayer games) to allow him to join the realm as described in this article:  https://support.xbox.com/en-US/games/game-titles/minecraft-realms-overview.

After changing those two settings in his account, the Minecraft game would crash on launch every time we attempted to open it.  

Rolling back the settings did not fix the crash.

Deleting and re-installing the app (after rolling back settings) fixed the crash, but it his local worlds were lost as a result.

I reproduced the crash a second time changing the same settings.

This time, I figured out I can fix the issue by deleting the XBLStoage.json file, in the Minecraft folder in the Files app on iOS.

After deleting that file, the game launches without crashing.  There is an error message about global resources being missing, but the game handles it without any obvious problems.