Kids should try the soap box derby out if they are interested. They should not be forced into it, but if it is something they see as fun they should try it. I think it is a good way to keep kids active and a good hobby for them to try out.
Kids should do this because it provides them with many positives. The kid is given a goal and has to work to achieve it. The child also can learn the value of losing and what it means to still take value from failing. While the derby isn't the best way to do this, it is still a good way to do so.