Seat protectors are NOT SAFE and NOT RECOMMENDED and will VOID YOUR WARRANTY on your car seat. Meaning in a crash if your child is injured, even if the seat fails, its your fault, they cannot be held liable, because the warranty is void. Its an aftermarket product, read your car seat manual and it will explain. The seat protectors don't allow the car seat to be tightened properly, and give parents a false sense of security in their childs seat being installed properly.
A seat protector will not prevent dents in the seat. And a seat dent is better than a dead kid, right? The dents in the seat are not permanent, they will let up after having the car seats removed for a day or two. And when it comes right down to it... is it that important to risk your childs safety?
Don't let your kids eat or drink in the car and there won't be a mess in the back seat. Food getting stuck under the car seats is what makes the most damage to a seat, any seat fabric or leather. A crushed up cheerio or cracker is going to rub on the seat. So if you don't allow eating or drinking in the car seats, it won't be a problem.
If you need help installing car seats and getting correct information, please visit safekids.org and finding a car seat check and installation station near you.
For what its worth, I had a car seat installed for my first child in my sports car with leather seats. When we sold the car, the dents in the seat let up after a few days of having the car seat out. The seat was installed there for over a year, same position without being removed, and the leather was perfect under it. It should be the least of your worries.