There is a valet spray you can get from Halfords that is really good. Spray on and leave for 30 secs and wipe off. If the stain does not come out on the first instance, repeat this process. It does work as I have used it on my interior.
Good luck! 
I can vouch for this stuff, cant remember what it's called but the bottle is yellow and green with a brush attatched to the lid. Spray it on, spread/scrub it round with this brush, leave it a minute or whatever then wipe off.
Works a treat.
Best thing to cure the child is patronisingly put a plastic sheet on the rear bench every time they get in the car, making sure to make a massive thing of it infront of their friends etc. Either that or tip a milkshake of their prized possession!
