We don't allow food in our vehicles (I cheat on that rule when I'm by myself) and only water, usually in reusable water bottles that go back in the house. Any trash goes with us when we get out. I'm pretty picky when it comes to a clean car, my wife not so much, so I tend to do the lion's share of car cleaning. I do a semi-full detail every month or two which involves vacuuming, dusting/ interior protectant application and leather conditioner for the seats (if applicable). People that ride with us usually remark how clean the interior is with four kids, but I don't think it's that difficult at all- just get the kids in good habits.
When we buy a vehicle, it's usually an older one with 100k miles or more-- it's quite the challenge to find one without the interior half destroyed from kids. Stains, rips, broken trim, you name it. Or around here, there's lots of smoked-in cars which is a deal breaker for us.