I have fond memories of family road trips as a little kid but maybe that was because we never did daily driving on our trips beyond the first and last two days to escape Texas (San Antonio). We did a day or two before moving on thereafter. Every year my parents picked an educational theme for our 3-4 week trip and we did similar places along the way.
My Mom was good at distracting driving games; license plate spelling games, sing alongs, cards, identifying birds from a guidebook, etc. We smiled and played along 'cuz Dad got grumpy otherwise, but it was the stops that provided the 'fun.'
Whether we followed Route 66 or old migration trails West, each leg was a story as seen through neat museums, parks, vistas, or places where a young mind could be transported via imagination to another time. Dinosaurs in Colorado, wildlife in Wyoming, Cliff Dwellers in NM, cattle drives through grasslands, or Native American lands (Canyon De Chelly), every destination was chosen for its offerings.
Dinner and a campfire ended each night with one or the other of my parents reading or telling tales of the next destination. Nowadays, you can get DVDs for the same effect. You can check them out at the local library. Old westerns like "Son of the Morning Star" before Little Bighorn Nat'l Park or use the net to find documentaries that can tell the story for you: The Mystery of Chaco Canyon | Watch Documentary Online for Free
Given the ages of your kids, maybe only your eldest would be engaged by a theme trip this summer. To my mind, less miles and more experience is better if you want to come back rested or restored by your time away from work. Whatever you decide, choose with your kids interests in mind to spark a family tradition all enjoy!