I'd say go with seat covers. Please please please don't think I'm suggesting your kids are like dogs, but on "Pet Edge" (
PetEdge.com Wholesale prices on exclusive dog grooming and pet supplies) you can find several designs and styles of washable seat covers at really reasonable prices. They have a camoflauge design that I'm thinking little boys might think is cool. You might have to cut some holes for the seat belts. You could probably also make your own seat cover out of just about anything, such as a blanket. You'd just have to sew on some elastic loops and cut some holes for the seat belts.