Tony,
Attached is a crude plan of the way my brother and I made them a bunch of years ago. The axles are held on by thru holes and nails or screws. You want the front axle to be wide so it can have a fairly tight turning radius. We used our feet for brakes on a paved road, mom was always getting mad cause we went through shoes so quick! The wheels are mounted to the axles with thru holes and bent nails and washers.
We used to have neighborhood drag races with these, no-one ever beat me pushing my little brother. We used to take a produce box with the holes in the side of it, and mount it on the front open side down, put a cut out milk carton on top for a blower, and paper towel tubes out the holes on the sides for exhaust pipes.
Man this brings back some memories. /forums/images/graemlins/grin.gif
Hope this gives you some ideas.