I'm with DougW. It's going to take you longer than you think.
Plan for around 250 miles a day driving time. It sounds slow, but by the time you get 2 rigs coordinated and on the road and make the stops necessary while traveling with children, that's probably the max you can do. It's going to take you around 6 days each way to get there and back.
This sounds to me like a destination trip, so you're going to have to find out exactly what your dad wants to see. He's obviously got some idea of which part of the Rockies he want to see. You could take the interstate to Denver and then head up to Yellowstone with the Grand Tetons which are very impressive mountains. Then you could head south toward the Grand Canyon and south to I40 for the journey home. Along the way should be the mountains you want to see.
If he wants a relaxed view of the mountains, consider a tourist train trip on whatever route you choose. They offer a unique view of the mountains that you cannot see from the road. https://www.colorado.com/articles/co...do-train-trips