I've travelled from Massachusetts to South Carolina by car with my nieces and nephews several times. One thing that I've done is have surprise wrapped gifts for the kids that I give them at every couple of hours. They were just small things like Mad Libs books, Invisible Ink books, hand held games, coloring books, and those writing things that you write on with a plastic "pen" and pull up the sheet to make it disappear--whatever I can find at my local dollar store or buck a book. I usually wrap each gift with a small amount of candy (a roll of smarties or a couple of starburst--enough to make them happy but not put them on a sugar high). The kids love getting their gifts, and it helps them pass the time. If they start getting fussy, we remind them they have another gift coming in x minutes, and they usually stop acting up.
One of my sisters had luck keeping her kids from getting too fussy in the car by telling them that if they would good, they would get some extra spending money.
On a recent car trip with my five year old cousin, we passed at least an hour by making up a story one line at a time (when you go around, and everyone adds one line to the story). We let the five year old lead the way, and of course, being a five year old, he decided the story had to be about a smelly fart man. But it kept in happy in the car for an hour, so we played along.
We also never travel without a walkman for each child, that way there's no fighting over what to listen to. My niece road round trip on the train from Boston to New York listing to the "Spirit the Stallion of the Cimmaron" CD and we barely heard a peep from her.
Have a great trip.
Karen