Give them some film and tell them to photograph what they see, you'll be surprised at the results you get back. It may be wise to give them a little help on how to use the camera meter to get in the ball park with exposure. Don't bore them with fstops etc at this stage, in my experience many youngsters find that a big turn off at the early stages. Encourage them to experiment and some of them will come up with some wacky images and that helps get them more interested for they are doing their own thing. Once they are hooked they'll come to you to find out what they need to know. If your experience in this is anything like mine you're in for a great time and I'd predict a few excellent and probably very different images from what you imagined.