Same here on IPad Air 2. Created a child account for my daughter, but she couldnt join realms or add friends on IPad. Finally found settings on xbox.com and gave her permission. After that Minecraft on IPad won't start.
Tried all workarounds mentioned, and when turning WIFI off I can get into the game for a couple of seconds, but then it still crashes.
Her account only has one friend, and thats my account.
iOS version: 12.3.1
Minecraft version: 1.11.4
Ok, finally found a workaround/fix to this. Applies to iPad, but might work for Android too (then replace steps 1-4 with corresponding steps for connecting to the android filesystem)
Connect your iPad to iTunes. I use Windows version. If first time, wait for drivers to download. Follow instructions.
Mine started to backup immediately. If so let it finish.
Choose "File Sharing" in the left column, then select "Minecraft".
There you will see a couple of folders and a couple of files. Optionally save the "games" folder to disk as backup of the worlds, I did this.
The file "XBLStoage.json" (file is actually misspelled... 🙂) is the culprit. It seems to store xbox live connection info, including a token that i suspect might cause the trouble.
Select "XBLStoage.json" and then "Save" at the bottom (might need to scroll the window to see the save button)
Open the file on your computer and ERASE EVERYTHING IN THERE. Then save the file.
Back in iTunes, choose "Add File..." at the bottom and select the now empty file. Choose "Replace".
Press "Done" in Itunes and disconnect the IPad.
START MINECRAFT AND REJOICE! 🙂
I suspect the issue here is that when the account information changes (as when we changed our kids preferences), a new xbox live token is generated and the old one is invalidated. The token mismatch might then cause a crash when trying to log in. Erasing the contents of the file clears this. I might be totally wrong though! 🙂
Hopes this helps anybody else.
/Martin