My son and I won the whole thing a few years ago (out of about 120 entrants). The first 2 years we weren't even close. I must of spent maybe 2-3 hours max on the one that won. The last poster was completely correct in that it is all the Dad's work and none of the child's. (See comment above about the first 2 years). So we did the "sharing the build" stuff for the first two years and I am sure he enjoyed it. The year we won, he didn't even really care because of the other poor outcomes, so I figured I would try to win it for him. I really think that after you get a few things right, there is a lot of luck involved. A couple of key points: Keep your kids hands away from the project, he'll only mess it up
Make sure the weight is as close to 5 oz as possible. If you don't have a scale, borrow one or buy it. Most cases, they will let you check it on weigh in, but it still needs to be close. Better to be slightly heavy and drill out the weight then add. Polish the nails so the axles have no burrs. Used powdered graphite. If you have a drill press, pre-drill all the axles before you shape the car. Give them a few degree's of negative camber evenly to all the wheels. You can have two cars that roll straight, but one can be rolling freely and another can be fighting itself with wheels that may have caster angles not perfect straight. Yadda yadda yadda. Internet searches will turn up a lot of this.
My son Kyle and me. He is no longer involved with scouting. It was fun for a while, but I found the whole thing a bit "forced". The parents sat around and talked the whole time while the kids mostly ran around unsupervised. I have more interaction with my son watching "The Simpsons". Hopefully your experience will be better. I am joking also about keeping you kid away from the project. If you want to compromise, let him do the design and painting, you do the mechanicals. Good Luck.