simple tank style control platform. teach them about gear ratio (speed vs. torque) and steering methods. then use the national semiconductor full H-brige all in one IC for motor control up to 2Amps continue (fairly good for small robots up to 25 lbs). you will have to teach them a bit about control of a DC motor, PWM, but it will be just the right amount. a bit heavy for 12 to 15, they may not understand all of it, but they will certainly appreciate the technology and "get the feel of it" which in itself is a major achievment. one thing leads to another..
to control it, you'll have to use an RC controller that outputs PWM + direction bit. other than that, you're good to go. cool robot with a good challenge rating for the age group with supervision.
good luck!
Mr.Cool