A potty in the car is a must. Make up a kit: potty, regular shopping bags or small garbage bags, and wipes. One great tip is to cut some diapers in half (most everyone has some "leftover" ones in sizes kids have outgrown) and use a half to line the bag making the bag much less messy and smelly.
Stop often. Offer them a drink a short while before you plan to stop to guarantee they will go once you stop. Success!
Most kids won't care if you put a pull-up on in the car to save yourself some messes. If they mind though, you shouldn't do it.
Enjoy the trip!