It's not really worth it. Here's the real secret... don't let messy things in the car.
No eating allowed. With a baby it isn't going to be much of a secret, but the second you let your 3 year old in the back seat with a tube of yogurt and a red velvet cupcake you can expect a mess that no amount of Scotch Guard is going to protect you from.
Trust me (three kids later) when I tell you that better than Scotch Guard are these things: Just an example, but you get the idea.
You'll get far more dry crud dropped from your child than wet stuff. The occasional wet froot loop might fall down there and then watch out. It will leave a lovely little stain. Since car seats typically don't go in and out of cars, the crud will stay in the seat for sometimes months. Getting something like that car seat protector, or at the very minimum use a towel under the car seat, is the only way to really protect the fabric. The real damage will come later when the kid is in a booster and much more mobile and brings in messier things. You've got two or three years before that happens, but when it does I suggest getting a vehicle with leather in it just to make your job easier. Trust me on that.