I have had people carrier full of kids for a while - if its heavily soiled little bits eg soda spills, ketchup and the like, I used Vanish foaming mousse to spot clean first( but deviated from the instructions by pre wetting the worse affected bits with a little nail brush ) let those dry. Then spray foaming mousse over the whole seat and work the area with Lidl micro cloth, then placing an old towel on the seat tops to soak up the dirt into the towel. It worked a treat! If it is very dusty vacuum a couple of times first to make your life easier.