I think all Amercians should try to make it to D.C. at some point in their life. Great place to take the kids, but only if they are old enough to understand and appreciate the history (I'd say at least age 11 and older). Not a place you really want to push a stroller around! It also helps to be in fairly good shape so that you can walk around, to get a real feel for the city and streets, although the Metro is also a convenient option as well and easy to get around from place to place.
If you have the time, write to your congressman and request tickets for a Capitol tour. Well worth it! I went with three friends and one of them was able to get a private behind the scenes tour of the Capitol and it was one of the highlights of the trip. We tried to get White House tour tickets too, but pretty impossible these days. You can try though, but do it MONTHS in advance!
Other highlights to definitely take in--Arlington Cemetery (a short ride on the Metro), The Basilica of the National Shrine (especially if you happen to be Catholic, or just want to see a BEAUTUFL church!) Of course take a day to see all the National Monuments and War Memorials. You can walk to all of them in a few hours time. The Smithsonians are a defiite must, but to really see them all, you need an entire day just for that.
Be sure to stay at a hotel in a good central location (near a Metro station too, but also near Pennsylvania Ave. so you can easily walk to things). My last tip is to take time to see one of the newest attractions in the city, "The Newseum" (this one is not free, but worth the cost if you want to see history from a news persepctive and learn a lot about how the Media works and remember so many events in our history). Allow at least a few hours to be able to take in everything here too. It's A LOT to take in! Six levels worth! Fortunately they also have a cafeteria downstairs if you need to take a refreshment break.
OH... one more thing. Go to the top of The Old Post Office observatory tower for an incredible view of the city, including the Washington Monument, which obviously you can't see from the top of the Washington Monument. You can also do some good souvenir shopping at the Old Post Office and grab a snack.
Above all, enjoy all the incredible history and take the time to read about things and learn! It is a trip you will never forget.