Omg, that's exactly what I was thinking.. My man's was told who to kidnap and given no other bits of info about them outside when it'd be most convenient to do so. He breaks in not knowing about the kid, but decides to help her out of kindness anyways. And he keeps it a secret, but she keeps bringing up her missing parents and his guilt starts to build up more and more.
Will he or won't he confess to what he's done? Will the girl gradually find out on her own about the facade he's putting up? If he was previously told to kidnap them, why would he then go to their seemingly empty house? If he quit the life of crime and is on the run from the law and a crime organization, is he putting her life at risk just by being with the girl in her search for her parents?
There's definitely a story somewhere in there!