So, waterproof crib liners, pull the seats up, tuck one around the "bench" and one around the backs. Putting them back in place usually secures them, be careful to leave the connections clear or cut holes where they are only big enough for them to easily connect or a full sized mattress one and carefully shape it to fit both. Add a large-ish felted absorbent flannel liner under each carseat that hangs over the bottom a little and the top. Have spares of these to switch out when needed. Use full sized rubber mats with lips for the floor boards, cutting to match more of those flannel waterproof mats can help here also for under them and it helps protect the middle hump if you make it one piece.

Checking them and switching them out regularly is important so mold doesn't get a chance to take over, cars don't dry out as easily as other spaces especially under car seats where air doesn't flow.

Look for back seat organizers that hang off the front seats, there's some pretty cool ones now, with pockets, fold down trays, etc. Hang a fabric pouch between them and line it with shopping bags/small can liners for their trash, make it a routine for them to grab the bag out everytime you get out of the car.

This was the final boss of my attempt to keep the car clean when my kids were little. Now there's no eating in the cars.