Multiple Grocery/Garbage Bags - There will always be something to throw away and can also be used for laundry bags to isolate the dirty clothes.
Multiple Rolls of Paper Towels - There will always be something to clean or substitute napkins.
Individual Kid Bags - Every participant in the car needs their own traveling bag. Make sure they put into their bag items for the car and the first 15 minutes after stopping so Mom and Dad do not have to immediately find something.
Cooler - Have each participant put their favorite drink and snack into the cooler.
Spending Money - It is easier to tell each kid here is your money for the trip and have them buy their own snacks or novelties rather than say no 25 times.
Wipes - Yes they will get chocolate and everything else on their hands and mouth.
Map - Make a simple map of the trip. Having them look at the mile markers may help, but you can never avoid them saying, "Are we there yet?"
Extra Clothes - If you are going to the mountains or plan on being outside at night, you will need jackets. They can be used for extra pillows and keeping warm for the air conditioning.
Medical Kit - Look into the medicine cabinet and put into a bag or kit everything that you normally go after including band aids, prescriptions (order them 1 week in advance), and the ever present aspirins (probably more for the parents).
Batteries - You can either go to the discount store ahead of time and get extra's or pay three times as much on the road. You do not want to spend your trip at WalMart.