I've learnt the mess comes from the missus, not the kid.
I feel your pain, mine is very good at carrying stuff out to the car, and that's where it stays! If I didn't do a clear out every week, her and the kids wouldn't fit it by Tuesday!
Although leather seats are a godsend, damp cloth and as good as new again. Currently looking at Sharans/Alhambras and can't get over how few came with leather.