The Iowa State Historical Museum in Des Moines is just south of the capitol. Admission is free and it's open Tuesday - Friday. There are two kid areas within the museum. On the 1st floor is Hands-On History and on the 2nd floor is Goldie's Creative Corner. Both of which are designed for kids 10 and under.
My kids enjoyed the Hands-On History area and I really liked the Hollywood in the Heartland Exhibit. The videos regarding Field of Dreams and The Bridges of Madison County were quite interesting. I would have liked to look at more of the exhibits, but my kids don't have a lot of patience for museums yet.