This isn't the "how do I win" answer you want, but my two cents on derbies (done ten years of them between my two kids.)
How to make it look nice- what you think looks nice and what your child thinks is his/her dream car is likely to be very different. Guide them to making a car they can be proud of.
We start out with a blank piece of paper. I fold it in eighths (half, and then quarters the other way). Let them quickly brainstorm ideas for what they think would be cool. Sky's the limit, these are just ideas. (One time, DS wanted to make one looke like the Eiffel Tower, lol.)
Then sit with them and talk about what ones would be practical to do (because of cuts or easier paint jobs, whatever.) Make sure you stick to your rules about extending the block in any direction. (Not sure what rules Awana follows, but I know scouts has these, mostly so it will fit on the track.)
Adult cut/sands the car. (The simplest shape is a basic wedge-slant cut the nose and and glue it on the rear end. Looks like a triangle of cheese. {and often time is painted in derbies to look like cheese, lol. Not necessary- any paint job will do!})
Use a spray paint for the main color of the car. (Yes, I let my kids spray paint. Practice on cardboard first to get the hang of it- steady pressure and constant motion. Nice even strokes that start ~before~ you get to the wood. Many light coats (with drying in between) better than a heavy gloppy coat.)
Let them decorate it however their little heart desires (here's where you just step back and let them use their "vision"

) Acrylic paint markers make this, well, child's play. You can get a pack of 4 or 5 of them for about $10 at
Walmart {craft dept} or any craft store. It's really as easy as just drawing with a marker at that point. They can also use stickers, or cut pictures out of magazines to glue on. Glitter and Elmers glue popular at girl scout derbies
There are a ton of links out there on how to sand axles and wheels. Do look at them to get an idea how to get them on straight so it will roll well. Make sure it is the maxium weight (5oz if you are following Cub rules.) You can buy weights at the scout shop or Michaels, can also just put screws in the back or glue on coins, whatever. The post office will let you use their scale. Be aware the"official" scale me be different and you may have to tweak day of the race.
But don't kill yourself trying to do every last "winning tip". Unless it's a small pool, odds are your child WON'T win. Stress that the fun of the derby is to do their best (to steal the cub scout line).