Check your community center programs. Montgomery/PG county run programs through MNCPPC at many of the community centers throughout both counties and have regular basic children's gymnastics. We started our twin boys in the 3-5 yo class when they had just turned 3. They enjoyed the class a lot, but they were not quite mature enough for it. The coach needed kids to listen and when it wasn't their turn, my kids would run around the room just being silly. The coach originally asked the parents to observe from the hallway (our community center room had class windows that the parents in teh hall could watch through. But after 1 class with our twins, she accepted my wife and I coming in and chaperoning the boys in the class. They still had fun, but we were able to keep them from running around laughing and just carrying on most of the time between their turns to try things. They're now 4.5 and we may try tumbling class again for them since they can follow directions a little better now.
But the class that we had was really good. She taught them how to do basic tumbling even for the risk adverse. Like there was a wedge shaped gym mat. She had the kids stand at the top, put their hands down between their feet, tuck their head down and then lean forward and they would somersault down the slight incline It was a very slight incline, so it wasn't scary. But just enough to help the kids start a somersault. And my kids thought this was fun.
We have tried a number of 6 week county run programs that have been great for us to experiment with classes and activities that interested the kids.