For something practical, placing a cut yoga mat on thw seats will work great, as long as you remember to clean under it every couple of months.
When my sister and I were younger, we weren't allowed to eat anything in the car that would make a crumbly mess (unless my mother road in the back to prevent a mess) and NEVER any drinks of color (juice, dark soda pop, etc). We were only allowed clear beverages. No ketchup, mustard, bbq sauce, nothing like that. And it has taught me a lot of respect for car. We weren't allowed to touch the inside of the car windows. The interior of my parents cars ALWAYS look(ed) nix nix that had taught me the same for my own.
We also didn't have DVD players and other entertainment until I was 15 or so, and we never had a need for it. You can save that money also, and tell your children to talk to you and enjoy the view out of the window.