Thanks for taking the time to post, I really do appreciate it!
That is a most interesting situation… Indeed, Migration Assistant should only transfer the files you specified, lest you ran into a bug, which is always in the realm of possibles.
You say it copied files on the hard drive but cannot access them… In that light, how do you know the space is taken by files associated to other accounts? Is there a specific clue or message you saw that pointed you in that direction?
Should other accounts have been transfered along with the one of your daughter, you should be able to simply delete them, as well as all the files they contain, by using the "Accounts" preferences pane available through the "System Preferences" application.
Let me know if this helps,