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