It seems that it would be easy to add a "Mini / YOUTH" class and leave it open to many bikes. I doubt that there would be more than 10 kids at any event to start with anyway, and leaving it open to all past mini bikes would give any sized youngster the opportunity to ride in the class whether he's on a 50cc bike like an Honda MR50, a 60cc scoot like the RM and YZ 60, an 80cc bike like the CR, KX, RM, or YZ, or even the rare 100cc bikes like the RM / YZ again. Then open it up to all the XR 75's, 80's, and 100's out there too, and you have plenty of bikes to choose from for Mini Riders to get out on the track no matter what they're age or size.
Seems to me that the riders of this youth class would be there to ride with their Dad's anyway and not necessarily chasing a championship or points battle like Loretta Lynn's bound youth on current machinery. Just let them ride and have a place on the track so the whole family can come.