That's a really good question. That's a pretty big age gap, so it's hard to tell what they're going to like. I've been teaching a couple of classes right in that range, a group of 8-10 year olds and another of 14-17 year olds, for the past few months and I've hit up everything from the standard Ka'au Crater Boys and Iz to Nickelback, Matchbox 20, and Natasha Bedingfield.
But I think the best thing you can do is ask the kids. It's really important for them to be able to play songs that they know and normally listen to, or else they might not see the point in learning and will lose interest pretty quickly.
As a first day starter, you could always bring in a song with the old reliable C, Am, F, and G7 progression with the down down up up down strum to get them acquainted, and throughout the class always sprinkle the music theory and scales and stuff they need to know; but after (or even before) the first day, it's good to take their temperatures to see where they're at and more importantly what they want. But yeah, that's about it.
Take charge and have fun!!