Yellowstone is big. Easily spend a few days there in the area. I recommend the Yellow Bus guided tours of Yellowstone. You see more and learn more. Yellowstone is so big they have two tours; southern half and northern half. The live bears and wolves in West Yellowstone are also worthwhile.
Also The Grand Tetons and Jackson Hole are just South of Yellowstone. Grand Canyon is good too. The kids need to get out and run around a bit.
Might consider the Black Hills also as you are driving through.
Good luck. Your time will go fast. Colorado is also a great place to visit.
