Simple Robotics to do with teenagers
You can try BEAM robots, you find tons of them when you google. They should be cheap and easy projects to build and understand. 

some of the link are :
http://www.instructables.com/id/Solar-Powered-Miniball-Wannabe/

http://www.instructables.com/id/How-to-make-a-mini-solar-robot/

http://www.robotroom.com/Appetizer.html

http://www.tombot.net/beamrobots.html

Apart from that if you want to include microprocessor then you can do some light chaser or line follower or obstacle avoidance which are fairly easy to understand and build provided you have prior knowledge about micro controllers.