A lot of good ideas on this thread. The best one being that the kids should be driving from the driver's stand and not from the track area. As always safety first.
Have you tried talking to the "expert" driver about the situation? It sounds like you are having a problem with one driver in particular. Perhaps you can work out a schedule with him to share the track? If he is any kind of real expert he is going to want to run one tank/pack then go back and wrench for 30 min or so to get ready for the next run. You guys could run during his wrenching time, and vice versa. It gives him free track time and time to wrench, and the kids some downtime to watch an expert drive and try to pick up some pointers.