I like the idea of heading to the Colorado Mtns in lieu of Yellowstone until you have more time. From the time I was about 5 years old my family headed West to Colorado--exploring different parts of the state each year and often stopping in South Dakota coming or going. Those long trips in a 11950's era non-air conditioned car across the hot plains of Kansas or Nebraska are some of my fondest memories of growing up. Dad and Mom would usually leave about 2 in the morning the first day, getting lots and lots of miles behind them by noon. They piled sleeping bags, pillows, duffel bags and blankets to fill the space between the front and back seat and we had one big bed in the back. By the end of the 2nd day we were in the mountains and usually set up for several days to just rest, hike and enjoy the area before exploring one or two more areas and then making the long trek back home.
I'm sure Mom and Dad were tired by the time we got home--but we had some wonderful times and those days instilled a love of travelling and enjoying the great out doors that have stuck with me for over 60 years. Go for it. Take lots of non-sticky snacks and be prepared with activity books, "contests" for the family to compete in while riding along, and yes, even some electronic games to help the kids pass the time.
Also--highly suggest picnic lunches in a city park along the way to give the kids plenty of time to run and play and be ready to take a nap when they get back in the car. We have used these ideas with numerous grand kids who have made the trek west with us--and they have all had a good time.
Go for it!
