I-90 basically follows the Lewis and Clark trail and a good starting point for that is St. Louis and the Gateway Arch. It has a great museum about the history of the expedition....and a ride to the top of the arch is something the youngsters would like I think. There are a few other L&C museums along the way too.
If you decide to go US-2 then head up I-75 to the U.P with stops in Dearborn, Frankenmuth and accross the Big Mac bridge. Stop in Mackinaw City and Mackinac island. There is a good maritime/shipwreck museum in Grand Marais and the Pictured Rocks National Lakeshore in Munising.
Take in the Crazy Horse monument and of course Mt. Rushmore in Rapid City SD.
I always order the travel guides from all the states we are going through, just go on line and order them as they are free. Lots of ideas and usually have area specific maps and they list campgrounds too. There is a good KOA campground at Devils Tower too if you take that in.
There is so much to see that you will have to choose wisely and manage your time well. Its a fantastic trip so don't rush it and see if you can engage the kids in choosing what they might like to see and do. Enjoy your planning.