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Go to the zoo. I thought it was kind of dirty when I was there, but the kids probably won't notice. Plus they have pandas.

Of course you should check out some of the Smithsonian museums. Air and Space is probably the best, but kids usually like the Natural History museum. American History is good. Skip the lesser-known ones. Skip the Halocaust Museum unless they're old enough to handle it. Definitely skip the American Indian one -- zzzz. The Spy Museum was decent.

Go to the National Archives so they can see the Declaration of Independence and the Constititution. You should also plan trips to the major monuments -- Jefferson and Lincoln Memorials, Vietnam, Korea, Washington Monument, etc. It won't take long, but they're major sights worth seeing once, IMO.

Mount Vernon is a good break from the Smithsonian museums and memorials. Oh, and a trip to Arlington National Cemetary is worth it. You can see JFK's grave and the Tomb of the Unknown Soldier (and changing of the guard).

I recommend doing a tour (they have drop-off trolley ones that have designated stops so you can move around at your own pace) or using the metro. If you don't want to pay the price of staying inside D.C. itself (and dealing with parking), you could always take the metro in each day. Just research the area a bit.