Here is what I've helped my 10yr old son with ( a bit ) off and on over the last month.
Tonight is the weigh-in. This morning we just finished putting on the steel ball-bearing weights, and we're at around 132/133 grams. The max allowed weight in our club is 140gm, so I think we're good there.
[aside: the weights were a big pain in the neck. When we finished the car, I was surprised to discover that it was only half of the allowed maximum weight. It was not easy trying to find about 65gm of weight in my shop that were not enormous bolts or such like. 1" diameter washers only weighed about 5 gm. If I start drilling holes for them, the car would quickly be in pieces! We found these ball bearings in a magnet-discover-kit-toy that my son just got, and decided to make them a feature instead of trying to hide them.]
Next week is the race. The boys club from our church compets against the girls club. Ear protection is almost mandatory, as the volume that 75 kids can create in a church basement is truly awe inspiring.
The design was my son's. I did the bandsaw work (the top angle) and he did the scroll-saw work (the side curves) as well as the sanding and painting. He chose the lightning bolt as well.
Even if we lose the race, I think he's already a winner.