Tracery is straightforward enough for kids old enough to read. It doesn't produce pretty pictures, but it might be a good fit for the older kids. You might thi

Tracery is straightforward enough for kids old enough to read. It doesn't produce pretty pictures, but it might be a good fit for the older kids. You might think of grabbing an OLPC machine, making a couple demos with turtle (like, lsystems or something else with knobs that are easy to tune), and giving them that.