If you are going to be in Montana, then pop north to the Canadian Rockies. The National Parks ... Waterton, Banff, Jasper. It's stunning and beautiful ... and different than farther south. The Columbia Icefields are between Banff and Jasper. You can park the car and walk to the toe of the glacier (the walk gets longer every year). And you can take a tour that goes right out onto the glacier. Very cool (literally). Lots of fun! Take a camera and sunglasses!
If you're going to be in TX (my parents did a 3 month US trip when I was 13, my siblings were9 and 10), Big Bend Park ... very cool. San Antonio and the Alamo. I learned SO MUCH US history on that trip! So much to see. It took us 3 months to get from CA to Washington DC and even then we missed a lot. This will be an incredible memory for your kids!