OS 6 and Game Center Login

After updating my daughters iPad with OS 6 update, we can no longer login under her username. My son has a iPad as well, and his GC username and account comes up instead.

I have tried logging in under her username ( Instead of the Apple ID) in GC with the password, but that still doesn't work. She is going to be devastated if she can't get back to her original games.

There has to be an easier way to Iogin. I appreciate any help, it seems that lots of other people had the same issue when they did the OS 5 upgrade.