We've been at it for 3 years now, and soon to start our 4th. I am a big proponent to let my boys be involved more than me. I help with the rough sawing and weighting, and help with the wheel/axle smoothing and placement to give them a fighting chance, but let them sand and paint their own designs. Unless they become auto body shop experts, they are just as happy sanding for 30 seconds with 100-grit paper, and painting it with poster paint. When they need important help on design and style, my daughter helps out in that department!
I have twin boys, so we have a minimum 2 to build every year, and my teenage daughter always wants one for the family race. I have wanted to build my own for the family race, but after helping with three sets of wheels, I'm usually done. Of all the one's we've made, the submarine sandwich my daughter made the first year for the family race was the fastest one -- she lost by a half-car length to an automotive engineer dad who really did make a cool frame-style car. The sandwich car surprised everyone how close it was to the lead car, since it defied most of the design considerations on weight placement and shape!
We have placed in the past, but there's one boy in our den who's dad makes two identical cars and races them the night before, then gives his cub the fastest of the two. He is undefeated in our den, and seems to have the magic of gaining speed every year. He admitted he pretty much does it all -- he wasn't even satisfied with his son's spray painting abilities.:frown: