Hi Morgan,
Not a dumb question at all. I applaud you for looking into this for you kid. Unfortunately, I am not the best on this either. Here is what I do know: I have seen a some karts utilize a basic engine (by Briggs & Stratton) around 5HP to 6.5HP (similar to a lawn mower engine) here in Oregon on our local circle tracks that clocked about 55mph almost in stock form. I have seen these kids race them & they are not allowed to alter the engines internally. So, the class/engine is 206 displacement, stock motor, on a chassis which is a small cart class called "Jr 1" for 5-7 year olds. Jr2 is 7- 10 I think. Look up a cart called "Animal" (I think Briggs & Stratton makes it). Do a search on Briggs & Stratton race carts or something like that & you should be able to see an idea of what they are, cost, etc...