Go through Wisconsin, Minnesota, and South Dakota on the way. The drive, even on interstate highways, is a lot more visually stimulating for youngsters than the zillion miles of apparent nothingness going through Iowa and Nebraska.
There are a lot of things to see along the way. In Wisconsin, you might want to stop at the House On The Rock
or some of the caverns. These are all west of Madison. North of Madison is the Wisconsin Dells area. The scenery in that area is superb.
I can't recall specific places to go through Minnesota, but it's a lush lake-filled state that you can easily enjoy.
In South Dakota, the Black Hills (and, of course, Mount Rushmore) is an area where you should spend a few days. Prior to getting to the Black Hills, there is the Badlands which is a must-see area. Stopping at Wall Drug is a must, especially for the kids.