It has been a few years since pinewood derby's. My son started derby running as a Tiger and ran a car every year through his second Webelo year. He did the work as age appropriate (the power saws were off limits to the tiger/wolf). Design, sanding, painting, and decorations were all his.
We have all the equipment a scout or parent could drool over ... and guess what ... Dad took the biggest ribbing when his cubby's car came in behind cubbys' cars who did not have dads (so guess who substitued as "dad"); the cubs were all treated the same ...
A couple of suggestions ... when guiding your cubby do not let the car be lowered, or narrowed ... cubs get more upset about their car not running the track than they do about not coming in first!
I have seen fire engines, green tanks, wedges, solid blocks ... painted, crayoned. Did not matter ... every participant got a "Derby Licence", Certificate, derby section, and ribbon. We gave out trophies and medalian medals; no scout got more than one trophy (if your car came in 1st, 2nd, or 3rd, it was automatically disqualified from best of show, most whatever, ... and the judges of the non-winner type trophies were the parents or guardians of the race winners! Never heard a complaint.
The creating the derby is the cake and icing ... the race is just the package.