I updated the last iOS 7 for my dhaughters iPad. We have had the same apple ID for iTunes shop but different game center nicknames so that we could play same games but saved separately with our personal game develoments. Now after the update the game center is not letting me to log her in with her nickname but is telling that this name is already in use!? What can i do to not to loose all her games?
A game center account is simply an AppleID.
You can go to https://appleid.apple.com and recover her password for the AppleID you want to use in Game Center. It might also be a password problem, so checking that the account is in good standing will give you better error messages and options to recover by using the website instead of logging in through iOS.