When I was about 7 I asked my Dad to help me build a go kart. Needless to say that never happened, so when I had my own sons I decided to fulfill that dream. I can't take all the credit as it's built upon plans but I deviated from them considerably by repurposing wheels from two bicycles as well as incorporated the crank and more. http://thegokart.wordpress.com
Take a look, and who knows maybe you'll be inspired. If you like it, let me know!