Peppy's vinegar method seems really cheap, so it's probably well worth giving it a go. The only problem might be if you (or your 9-year-old) can't stand the smell of vinegar! (If someone, sitting on the opposite side of a restaurant to me, puts vinegar on their chips, the smell makes me want to throw up!).
An alternative solution, which won't leave your car smelling like a chip shop, is to head to your nearest branch of Pets at Home. No, I'm not suggesting buying a cage for your 9-year-old ;-) They sell this stuff:
http://www.simplesolution.com/products_uk/stai n&odor.htm
It costs a few pounds but it's absolutely wonderful for completely destroying any organic odours. (I discovered it through owning an intact male cat who liked to leave his mark on my settee but I've since used it for eradicating all sorts of smells, including that of human vomit).
Chris