In terms of actually learning C, I'd recommend hitting up something like Topcoder that has a bunch of tiny challenges to do. The 250pt challenges are about the same programming difficulty you'll encounter programming the robot, and give you something to work towards as well as objective accomplishments (say to them "each of you complete 5 250pt practice room problems"). You can view other people's code in the practice rooms, so if you're ever curious about syntax there are lots of examples.
Basically, I find that in order to learn, you need something to make. Basically anything will do, but Topcoder supplies the 'something to do' without it being too difficult.
http://www.topcoder.com
Actually programming the robot on a basic level isn't terribly difficult. You just modify the values of some variables (pwm01,pwm02,pwm03...) between 0 and 255, and like magic the motors turn.