I don't agree with your son. I do not think the cars fabric is too busy to be next to the star. It is hard to tell for sure without auditioning it. But I think a narrow border of the orange say 2 to 3" then a very narrow white like 1" finished then the car fabric would be great for a 3 year old. It would result in a much wider quilt though. Your center is already 38". If you are looking to finish at 40" wide that only allows you 1" on either side. So I would rethink you finished dimensions of only 40" wide. Perhaps a 60" by 90" would look nicer with multiple borders. That car fabric needs to be at least 6" wide judging by the picture and size of print.
I think it will make your Carpenters Star sparkle and brighten up as well, that polka dot seems to dull it out just a bit. Use the polka dot for the backing and it would make fun binding too.