Elementary aged kids are probably the prime audience for the Museum of Science and Industry. Old enough to start understanding and engaging in the explanations and science behind the exhibits, while young enough to still really enjoy the playfulness in many exhibits.
The U-505, a captured German submarine, is probably the top exhibit, and this age will be old enough to appreciate the entire exhibit leading up to the submarine itself, understand the danger of U-Boats during WWII, as well as the drama and daring involved in capturing the U-505. Towards upper elementary, you may want to consider doing the on-board tour (purchase them as an add-on to your regular ticket); early elementary will probably still not be ready for the tour which is cramped and involves some scary stories and sounds inside the pitch-black submarine during one portion of the tour.
The Henry Crown Space Center is also a great place to visit, with the capsule from Apollo 8 and a practice lunar lander, along with several interactive exhibits.
Science Storms is popular at this age, especially the giant "tornado" and artificial lightning on the ceiling (warning: the lightning is LOUD). The Boeing 727 on the top level will be fun for kids, and You! The Experience has some fun interactive exhibits about the body.
The coal mine tour is targeted pretty well towards this age, although it's fine to miss it. The Numbers in Nature mirror maze is fun and surprisingly challenging. The transportation gallery including model trains are fun, as are the nearby baby chicks in Genetics. On the lower level, Farm Tech can be fun, especially for early elementary students.
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