Boys need something to do at all times. That is the best way to keep them out of trouble. Building a pedal car can be a good way to keep a boy busy for the summer, and to get rid of some old bicycle parts that may be lying around your shop or garage.
The seat must be your first and foremost thought. All of your power comes from your body pushing against the seat. Mount it in a comfortable place (unless it is adjustable).
Attach your front wheels to the frame. These are rigid mounted as your steering will all be done from the rear axle.
Draw a straight line between the point at which the front wheel touches the ground and the center of the car's rear axle. Repeat this same process on the car's opposite side. Select a point 150mm or so back down the lines. This will be the point at which the steering will be linked. Attach the steering arms to these points and link them together with swivels at each end to keep your wheels in track.
Use a steering column and drop it through a cycle drop tube. The steering column should have an arm attached at 90 degrees. Attach the steering column to your steering arms.
Use a lay shaft to be able to transfer drive from its center to the outside of the car. A chain sprocket in the front drives a chain to a five speed freewheel and a dérailleur located in front of and below the seat. The freewheel should be attached to one end of the shaft and a cog attached in line with the rear wheel. Attach a chain between the freewheel and the cog.
Bodywork is really up to the builder. The important thing to remember is you need to build it up enough that clothing cannot get caught in your gears or chains, and no parts are protruding that someone could get hurt on.
Install cycle caliper brakes mounted to both rear wheels. Brake levers should be mounted on the steering unit with the shortest possible cables.