We still have a small issue with parents building the cars, but I have almost completely put an end to it.
We allow the use of the "kit cars" sold at hobby lobby, etc. I would rather see a Tiger open one of the kit cars, assemble it, paint it, put the weight and stickers on themselves then to see a parent build the car for them. Lions and Tigers using kit cars are fine, by wolf they should be building their own.
How I get parents to stop building the Scout's car? We host derby workshops, if the Scouts get to see other scouts building their own cars they encourage their parents to let them build their own. We encourage younger lion through wolf moms and dads to build their own cars and we will even supply them. I teach parents to build the same car as their Scout. This way the parent can make the large cuts on the body of the car, then demonstrate to the Scout how to do the finer things like sanding, polishing axles, etc on the parent's own car. Let the Scout do the same on their car. The Scout may not want to do everything like getting weight placement 100% correct. When mom's car is faster then next year the Scout will pay more attention. This works wonders. Some of our workshops are on weekends, some are during the normal Den Meetings, this way the Scouts get to see others building their own cars.