Depends on how far you want to drive. The obvious is the museums (natural history is great for the kids, especially if they are into dinosaurs and bugs; if you have girls they will love the diamonds!) cannot forget the FBI/SPY museum and the zoo.
If you want to drive a couple hours, try Busch Gardens in williamsburg or head into williamsburg/jamestown and explore some amazing history (if you go and you like peanuts go the The Peanut Shop!).
There is also the Baltimore Inner Harbor (home of the Orioles)--aquarium, maritime museum and so much more.
If you can make it...there is an ice cream shop called Milwaukee Frozen Custard (it is not like baskin robins or cold stone) in Virginia, worth the trip!
A lot depends on the age of your kids. Besides the standard Smithsonian museums (Air and Space, Natural History), the National Building Museum can be fun- on some weekends they do a lego building hands on exhibit. Other good Smithsonsians for kids are the Air and Space Annex in Chantilly (there is a shuttle for the main museum) and the Musuem of the American Indian.
If your kids are mature enough to deal with it, the Holocaust Musesm is amazing- they have a special section for kids over 8 (don't take your kids if they are younger than that).
If your kids are into other countries, the National Geographic Explorers Hall has fun things for kids to explore.
Many kids like the FDR memorial- I think maybe because of the water features.
If your kids like the outdoors, a day trip to Great Falls Park for hiking/rock climbing would burn energy.
If you want to catch a show while you are in town, the Kennedy Center normally has kids programs and Imagination Stage in Bethesda (can reach via metro) always has good shows for kids.
Went there last May; just me and my wife, we are in our 30s. We had to stay away from the more visited sites due to so many school groups overwhelming the sites. We liked the Art museum (I surprised myself, but not really for kids), Lincoln and WWII memorial at night (although may be cold depending on the time of year), and the Holocaust museum was very enlightening. I can say that you should stay away from the aquarium at all costs, as it was very pathetic.
P.S. Brett Rules!!!
Definately you should go to the FBI building tour, the Mint Is very interesting, and also the spy meauseum. Those are ones that you and your children will love. The space meausem is interesting. There are so many to choose from, but these are ones that the kids will be happy with.
Pack a picnic and go to the Cherry Blossoms if you are going in April. that is a sight for sore eyes.