Snacks during your rest stops are always a good idea. They can mean the difference between eating at a place that looks great, and stopping out of necessity because everyone's famished. Also lots of small snacks keep everyone's blood sugar at a steady level and therefore less likely to be cranky.
The key is to choose snacks wisely. If your children never eat celery at home, they probably won't want it now either. On the other hand, vegetables with a fun dip can be great on a trip, if your kids would have enjoyed it at home. We avoid a lot of salty snacks because they cause children to be too thirsty. Grapes are a favorite, but we cut them in half for the little ones to avoid choking. (I cut them when I pack them.)
Sandwiches work well. Our favorites are peanut butter and jelly or cheese on very fresh bread. I make them the night before, flatten them down with my hand, and cut them with cookie cutters. Then I put all of one kind in a plastic box and refrigerate until we're ready to leave. This eliminates yards of plastic wrap and makes for a more appetizing presentation. Please remember though that cheese and dips need to be kept cold on your trip, too.
Juice and chocolate milk work well in the drink department, but ice water is also a welcome choice. I fill a plastic jug with ice cubes and then add as much water as will fit around the cubes. The water stays very cold for several hours this way.