When I was doing FLL, this was by far the best example of explaining the principals of line following using PID. I did this with my son in either 5th or 6th grade, and since then have used this as an example with our high schoolers in FRC.
In FLL we ended up with just a P controller in the competition code, but we spent about a week that summer doing this after work in the basement.
http://www.inpharmix.com/jps/PID_Con...ms_Robots.html