That will be a challenge. I have a 6 year old (7 next week) that I tried this with about 9 months ago. I put an 88 kit with TB race head on it. I also bought zokes and a red Baron swinger. lotsa bling. He is way, actually WAY faster on his KTM Pro Senior. The KTM/Cobra/LEM/Polini is the way to go for sure if you want him to be fast.
He still likes to ride his Honda but his KTM is his race bike. The KTM really builds confidence. The first ride is a little tough to get used to (Very snappy) but once the kid gets used to the power the suspension, light weight bike, and fast bike with out having to shift will make your kid quicker.