It's a big challenge to keep kids happy these days and I'm afraid the planned trip may not do it.
Unless you plan to do some hiking most of Yellowstone is viewed from the vehicle. You can walk over to the viewing area benches for 45 minutes until Old Faithful erupts and that is it. IMHO Yellowstone is better realized and understood by taking the two bus tours BUT that would not be of interest to kids even tho yours are special.
Mt. Rushmore is worth about an hour of time.
The western parks are wonderful but more wonderful for grownups.