I love the part where you say that you can't afford it, generally people with money problems often don't understand even if they have the money to buy something, buying it would be a "financial" mistake. The "funny" thing is that there are people who, without having the money they buy them with credit :S
When I was growing up I always reproached my mother (when I was a child and I wanted to wash the dishes you told me no, now that I'm older and I don't want to wash the dishes you tell me to wash them) so I think it's great that you give your son the opportunity to wash the car, I believe that children should not be discouraged when they want to do a productive activity just because of their "age."