I know Yamaha makes a 4 Stroke 50cc race bike. That's what most parents start their kids in racing on. You can also go the 2 stroke route. Early 90s to 00 models from Suzuki, Aprilia, and I think Honda make 50, 125, 250cc 2-stroke street bikes that are really nice and somwhat controlable....the only thing is they might be a little pricey and hard to find a dealer. Once your kids can ride a 2-stroke, they can ride anything!