Shutting down after clicking on world

My son has had minecraft on his iPad mini for over a year. He hasn't played it in a while and tried open it today. He can get to the main screen but when he clicked on one of his worlds, it would say "retrieving world" and then go to the main page of the iPad, basically shutting minecraft down. I had him uninstall and reinstall which wasn't a good idea because he lost his worlds. But it is still happening when he opens it and clicks on building a new world. So basically he can't do anything. Please help! Thank you