You'll want to ask for details in the
Airport Express forum, but basically what you want to do is set the AE base station so that only her computer can connect wirelessly to the AE at all. That's good policy anyway and will prevent anyone else from using the printer unless, of course, they unplug it from the base station and connect it to their own computer, something you won't be able to prevent unless your stepdaughter locks up the printer.
Regards.