A good place to start would be to go into
Settings and choose
Data Usage on your daughter's phone: if you scroll down, you should see the apps that are using the most data (assuming your daughter's the one racking up the data - if that's just a guess, it might be worth checking the other phones on the account as well).
You'll probably find it's a sync issue .. probably on something your daughter's started using recently like Facebook or Twitter
Try turning off Sync (using the Power Widget), then going through all the apps on your daughter's phone that are using lots of data (e.g. e-mail, facebook, twitter, news & weather apps etc etc) and checking any sync options. Either turn these options off altogether or drastically increase the time between syncs.