Do NOT put anything under your car seat, it will reduce the effectivness in an accident.
Leather is extremely durable, and the seat will bounce right back when a carseat is removed, usually a day or two and the seat will look like new. So don't worry about the seat becoming permenentely indented, it will NOT. So please do not put anything under your child's car seat. Even if you get a liner or a towel or one of those 'seat protectors' (which are not approved, by the way) your seat will STILL get indents from a car seat because you need ot tighten it down, and no blanket or towel or seat protector will stop that. So don't take the chance.... nothing under the car seat.
AS for them putting feet up on the front seats, www.onestepahead.com has a thing that hangs over the headrest of thefront seat and its plastic, and it protects the front seat from feet marks. Seems like a good idea.
EDITED TO ADD- that I hope that anyone using something under thier carseat would have it inspected by a CPST and NOT your local fire station or police department. A CPST (Certified passenger safety technician) gets ongoing extensive training on child passenger safety, where a cop or fireman only has a few hours of training once. No good CPST will tell you that its ok to put something under your car seat (some will say a shelf liner is ok... but you wouldn't want that on leather, it would melt to the seat in the summer).
I had leather seats in my sports car, and the carseat left dents for about 1 day, then it bounced right back... no damage at all, I traded it in for my SUV and it was totally fine. DON'T WORRY ABOUT IT!
DO NOT USE ANYTHING UNDER THE CAR SEAT!