The Smithsonian is fabulous but can be overwhelming with kids--there's
so much to see. But the Natural History and Air and Space will keep
them interested, probably the American History too.
A couple of other things that weren't on your list but are very nice--
National Zoo and the Holocaust Museum. My youngest were 9 and 11 when
we did the Holocaust-- I was concerned that they would freak out but
the presentation was great and the kids really found it powerful and
interesting.