Annam,
You are planning a disaster.
First, the road trip from Pitts to Des Moine is 13 (at least) road hours. Unless you have multiple drivers and a very comfortable vehicle, that is two days by itself. If you are including that in the two weeks, then you have about 10 days to cover approximately 3500 miles.
This will be a time you remember as frustrating and a a waste.
Now, Try this...
First stop - look up Ashfall state park in Nebraska. It is about a day out of Des Moine.
The Wall SD - Watch the signs for Wall Drug. It is a tourist trap with free water and cheap coffee, but you can't go by it.
Then on to Mt Rushmore and Crazy Horse.
Devil's Tower would be next.
From here, you will have to cross the Bighorn to get to either Teton or Yellowstone.
Going to Yellowstone, you have to go through Cody, stop at the Buffalo Bill Museum.
Now you go into Yellowstone. If you don't spend three days there, it will be a complete waste. Remember that YMP is larger than several eastern states. For your family, you will need a Full Hook Up (FHU) campsite. The only in the park is Fishing Bridge. This would be a bad idea for you unless you plan to book all the tours with the buses that run in the park. You need to study up on YNP. Take walking shoes, back packs and water bottles.
You might also be able to find an bivouac in West Yellowstone and rent a car to tour the park. It is not a place for a normal motorhome to tour.
Do you realize that I just burned off your first week and haven't even thought about getting you home??
Matt