What's wrong with the "youth" class. Here we have Junior A & B. They go fast, ride full size dirt bikes, have a lot of fun, get to ride the better time slot, more trophies per rider than C class (I think). Oh, did I say they go fast? Don't be concerned about the lack of competition.
Keep in mind that you might not be allowed to move "down" in class once you've ridden in a "higher" class. Like LWilson250 said, ask around. Although you may have to get a rider number before race day along with the right background and numbers.
Good luck on your first H/S! Keep your eyes open, have a ball!