But in a nutshell, you want to reduce friction and maximize the weight of the car. Those two things make for a fast car.

You reduce friction by removing the burrs under the heads of the nails and polishing them smooth, and by making sure the car runs straight (so it won't rub against the side of the lane on the way down). You can also reduce friction by making one wheel slightly higher than the others (so it doesn't actually spin). And of course, use graphite or some other dry lubricant.

Make sure the car weighs right at the legal limit. There is quite a bit of debate on the placement of the weight, but as a 3 time champion, er, I mean as the dad of a 3 time champion, I'm telling you that you want the weight as far back as you can put it. The very fastest cars remove a lot of wood and replace it with lead weight at the back of the car. But don't put it much behind the rear axle, because that might make the car bounce down the track. (If you can move the rear axle back, do it, but many packs don't allow that.)

Oh, and make sure your boy takes the lead in the design and painting of the car, and have him do as much of the building as he can. The ideal is for you two to work on it together, not for you to take it over or for you to just send your boy out to the garage.