A.V.
There's LOT to do in DC.
1) Even if you are staying in the suburbs, drive to Metro and take the train. 2 children under 4 are free for each paying adult. Get each paid rider a SmarTrip card - cheaper than paper fares and most Metro owned parking lots require SmarTrip to get out.
2) Hit the Smithsonian museums (free), but don't eat there. Walk a couple of blocks to the Old Post office, which is basically an old post office (you can go up in the bell tower) converted to large food court. Much cheaper and better food.
3) If you can swing it, hit the Newseum and/or Spy Museum. Those are paid attractions, but very neat.
4) Don't go to the DC aquarium. Take the trip to Baltimore's Inner Harbor and do the aquarium there if your kids are big into fish. I really like it.
5) Consider a "duck" tour. WWI (or II? I can't remember) amphibious vehicles that play music and are usually a lot of fun for kids. They give the riders quackers.
6) Go to the zoo! Free, and a lot of fun for the kids. There is a bird room where the birds fly freely around you and make the walk down to the Amazon Trail.