The easiest way to maximize the potential energy in your car is to make it as heavy as possible. Find out what the maximum weight is (usually 5 oz) and make sure you are right at that weight. Use an accurate scale and weigh your car throughout the build and design process. Usually, additional weights will need to be added to your car. Tungsten is the recommended material for weights (denser than steel and safer than lead).
Put the weight towards the back of the car. On an inclined track, this will give you a little higher center of gravity, which means a little more potential energy. Look at the car in the first picture. Most of the weight in it is in the back rear of the car. Just be careful not to put so much weight back there that it causes your car to do a wheelie.
One commonly used method is to make the car just a little heavy, then remove material at the race site by drilling a little wood out of the bottom of the car. This is the method we used last year.
The next tip that we'll share is axle location. The block comes with pre-cut slots in which to mount your son's axles. However, I would not recommend using them as they are usually not square, meaning they will cause your wheels to be ever-so-slightly out of alignment, which could pull the car to one side or the other and slow it down, especially if the wheels ride against the edge of the track.
Instead, drill new holes just outside the grooves. A #44 bit is the perfect size, but may be hard to find at your local hardware store, so I'd suggest ordering it online. The new hole needs to be drilled in perfectly straight, so either use a drill press or a guide (like the one below).
On a side note, our pack had fun, inexpensive, and easy to make trophies. They were small blocks of wood with Matchbox cars on top, spray painted gold.
Well, I hope you both have fun making your son's car. Remember to involve him in every step of the process. You can help him do most of the work while ensuring that things are done right. Be sure to explain why things must be done right and the benefit of taking time to do so. In my opinion, it should be fun for the boy but a little bit hard too, so that he remembers the effort that it took to build. That way, when he sees his car going fast down the track, he can equate hard work with positive results. Winning isn't everything, but teaching your son that research, correct application of principles, and hard work creates positive results is a valuable lesson.
blogging at So I Married a Craft Blogger