At Devil's Tower, make sure to stop at Prairie Dog Town. The kids will love it. Also, if you've never sniffed a Ponderosa Pine in the daylight, they smell just like butterscotch candy. Something about the sun shining on them brings out the aroma of the sap. It's just an aroma, don't start licking the sap. Anyway, they are all over out west, but in particular they surround Devil's Tower.
Then to Deadwood with a stop at the Mount Moriah Cemetery on the hill. Perhaps hit a tourist trap or two in town.
Then down thru the Black Hills to Mt Rushmore. You might enjoy all the other stops so far more than this one as Rushmore is packed and over crowded, but it's still something to see.
Then down thru Custer State Park and the famous tunnel with a view of Rushmore.
If you've never been to the Badlands, you might map that out and see if you'd like to add that onto your trip.