I rode on of those things once -working at a Yam dealer and nobody else could even fit on it and they were roadracers anyway. It was the ONLY bike I've ever ridden that I could not ride a lap without falling off. It was SO SMALL and SO PIPEY that I couldn't get enough weight forward to keep the thing from wheelying(and not steering) everywhere. Incredible little beast but your 6 year old had better have some cojones.
Go fast. Take chances.
Mike S.
