My son wanted to connect with his friends in Minecraft PE on his iPad, so we went through the process of creating a Live account for him, which we added as a child account to my Live account.
(Then the app told him that his privacy settings needed to be updated to be able to add friends, which we subsequently also did.)
Then the app crashed. Now, on relaunch, it crashes on startup. Every time.
How can we fix this? My son has put in hundreds of hours building worlds in this app, and now he can no longer access them.
Wait for Mojang to fix the bug, no one else can help.