Be prepared for the possibility that they will wake up every time the vehicle stops moving. If you plan to stop for fuel at night, remember that gas station lights are pretty blinding; side window shades are a very useful thing to have to cut that down. (Also, if the children will be in the back with an adult between them, one of them will be exposed to a lot of sunlight during the daytime hours, kids can get a nasty sunburn that way if the side window isn't shaded. Heck, *I* have to drive with a piece of white fabric over my legs in summertime on long drives, because if I'm not wearing long pants I'll get burned.)
Having them sleep 7 hours in the carseat with an adult sitting next to it shouldn't be a problem as an occasional thing, but sitting in the same diaper for that long really isn't good. (Dry-wick diapers don't keep a kid comfortably dry as well in a carseat as they do in a crib, because they are sitting in a semi-upright position and gravity causes the waste to pool in one spot. I recommend always using overnight diapers on road trips to prevent leaks, but they still need changing as often as you can.)
The PP who noted that stopping after a long drive so that adults can rest after kids have been sleeping for a long stretch was correct: that usually doesn't end well. I can still remember a situation 20 years ago when we did this; DS woke up when we reached the hotel at 1 am, and perked up immediately because he was in a strange place. 2 hours later we ended up child-proofing the room as best we could and putting on cartoons for him to watch, because he was super-buzzed, and we were totally exhausted. We woke up at around 8 to find him zonked out on the floor in front of the TV. We took him for a swim before getting back in the car.
Which brings me to the best tip I know for road trips with young kids: swimming pools. Locate them ahead of time and make it a point to use them. Swimming has a curious effect; it relaxes adults but exhausts kids, so it's the very best thing you can do to get the kids ready to sleep when you need to sleep, too. However, it means not rolling in at 2 am; most hotel pools close no later than 10, and often by 9.