Some Possibly Worthwhile Stops
Michael is right in that the beauty of RoadTrips is that there is simply no one-size-fits-all list of "must-sees" that everyone has to get to. In your case, with an interest in history and geology, and your stated route, here are a few places that should suit you. The Lowell Observatory in Flagstaff where Pluto was discovered; the Grand Canyon, of course, if you haven't already seen it; Petrified Forest/Painted Desert in eastern Arizona; Petroglyph National Monument outside Albuquerque; Fort Reno in El Reno, OK; the Oklahoma City Memorial; the Arch and Museum of Westward Expansion (together) in St. Louis; Mammoth Cave National Park in Kentucky; and Abraham Lincoln's birthplace and boyhood home (separate) outside Hodgenville, KY. You should also give your kids a map with your route marked out on it and let them see if there's anything near where you'll be going that they want to see.
AZBuck