We did a 3 week trip five years ago to the Badlands, Mt. Rushmore, Yellowstone, Tetons, Dinosaur Natl. Monument and then back to Atlanta. Our 5 kids, ages 1 to 10 at the time (and us parents) LOVED it and are begging to go back.
Some highlights other than the parks were the missile silos in S. Dakota, the prairie dogs at the Badlands, and the lake beach (smooth rocks) near the RV campground in the Tetons.
I truly believe that our kids will remember it for the rest of their lives. I know the then 1 year old does not remember, but when she sees the pictures now she wants to go again.