I believe MA charges extra if juniors are on the race schedule. However, if the juniors are having a "recreation ride" only, the cost is reduced I think.
Letting the juniors have a ride is a great thing for the future of MX as a sport and it helps the club to hopefully at least break even for running an event. Even if the kids are on modern bikes, some of them might develop an interest in old bikes.
VCM here in Victoria has a pretty good formula for making an event worthwhile to run. They not only allow minibikes, (mostly XR Hondas) but they also allow the pitbike lads to have a run as well. The Pitbike boys love having somewhere to race and put on some really good racing. Then we have the big bikes, Pre70, Pre75, Pre78 and Evolution classes.
I feel like a rider gets good value for money at a VCM race day, especially if you have more than 1 class of bike.