I always end up yelling, threatening to leave one of them at the rest stop, or crying (for sympathy). Can I do anything in advance or en route, to prevent emotional meltdown?
First let's take care of the pragmatics. Pack eight hours worth of individual food treats, games, music cassettes, toys, etc. (let your kids select these). Make frequent pit stops (every 90 minutes or so) to refresh and "air everybody out". Check your own levels of anxiety and apprehension concerning the trip and your stay with your folks. If you can't bring a relaxed comfort level to this trip, don't expect your kids not to pick up on your tension and mirror it in their behavior.
Now to the core of the meltdown-blocking strategy. How you frame this trip before you leave will largely govern its success and your children's behavior. Here's a typical parental mindframe: "Look, this trip is going to be long and stressful and I know no one wants to be in a car this long. I don't want any of you acting up and making it any worse than it has to be. I don't want to be in a bad mood when I get to Grandma and Grandpa's. Understood?"
Here's the different angle I mentioned: "I know this is a long ride but we're going to turn this into the holiday ride we'll never forget! We're going to do some goofy things we've never done before. I've got some surprises up my sleeve. Mrs. X (refer to someone they know) said there was no way we could turn this ride into a good time. I said you guys could. Was I right?" The first preview is an introduction to a horror movie, the second is an unusual, disarming, intriguing challenge.
Now you brainstorm your goofy journey. You kick it off by saying something like, "I packed some disguises for us, even Y (dog's name). We're going to change identities, names, the way we look at every pit stop. We'll make up different stories of who we are and where we're going. I'll tell a stranger at each pit stop part of our new story: Hi, I'm Lupe Valdez, my boys and I are on our way to Brazil for the holidays. Now, I need some ideas that are crazier or funnier than that for all our other pit stops." Once one of your boys comes up with one, the others will try to top it. Pretty soon the trip from hell starts becoming "Bill and Ted's Excellent Adventure" (in fact watching that movie for inspiration isn't a bad idea). Let the silliness and excitement set the tone for your gang to further develop this "adventure blueprint". "Let's call Grandpa at one pitstop, I'll disguise my voice and tell him he won some contest." "Let's play an imaginary game of baseball in another pit stop- in slow motion." "Let's make a ribbon and award it to someone at one of those pitstop fast food restaurants for having the best pants." And on and on.
"Hey, hey, we're the Monkees, we like to monkey around ..." You get the idea? The combination of encouraged and sanctioned family silliness and your issuing the challenge from Mrs. X (kids love to prove grownups wrong) that there was no way your trip could be fun, could well result in "My Family's Excellent Holiday Roadtrip". It will, at the very least, be a trip to remember. Are you game?