The big issue for you is the long, boring drive from Illinois to Colorado for a 1 year old and 4 year old. That's a long time for children to be confined to a car seat. If it were me I'd plan on no more than 4 or 5 hours of driving per day. Everyone will be in a better mood when you stop. Plan on places with a pool to wind down and relax.
Pick a location such as Colorado Springs and spend your vacation doing things in that area. Traveling from place to place will not matter to the children. A zoo is a zoo. A trail is a trail. Google the city for things to do with children. You'll get a lot of ideas.
Have fun planning!