In a moment of weakness I bought this:
http://www.homedepot.com/p/Monster-M...80BR/206734879
The plan is have my son follow my bro in law around a track I'm building. My bro in law can drive the bigger kart and my son can drive the Monster Moto to learn the track lines. My Bro in law is a very good Porsche Club Driver. We go to the local Racing Kart track and I just try to learn by following him.
Then when my son can handle the Magnum 125 by himself he will graduate to it. Until then he will solo the Monster or have an adult as a passenger in the magnum. This is turning into a contagious disease. My neighbor just bought 2 karts for his 4 and 3 year old. The 3 year old is way to young but it is waiting for him when he is ready.