Hi, Mutiny92 - What a great trip - I'm jealous; I love Washington, DC!
Here are a few ideas - first, maybe spend a bit of time talking with the kids about history, so they'll have a context when they get there. Here's what I'd see with them:
Smithsonian Air & Space & Natural History. Not exactly out-of-the-ordinary, but kids love both buildings. They'll see space capsules & can climb around in things on some displays, and they'll see dinosaur bones in Natural History. Then go by the American History area & see the Ruby Slippers from Oz.
Mount Vernon: Can get there by bus tour. It's a wonderfully preserved & restored example of life on a working farm in the early days of our country. They will be surprised at the work it took to live daily life, and how very small some of the rooms are. The line can be a bit long, but you'll enjoy visiting with the other families waiting to get in, and it moves quickly. There are grounds & outbuildings you can visit, and a gift shop.
Newseum: A museum about news and its history. Has old printing presses, some fun interactive displays (kids love them) and takes you from the earliest days to TV and modern technology.
Alexandria VA: A beautiful town - easy access from DC. Take a walk down the main street, which slopes down to the dock area. Look at the cute shops (and old architecture) and eat some great crab cakes & fries.
Boat Tour at Sunset: They take you up & down the Potomac as the city lights come on and explain what you're seeing. Probably not as hands-on informative as the bus tour (which was worth taking), but kids will like being on a boat.
Korean War Monument: If you see one monument, I'd see this one (aside, of course, from the Vietnam Wall and the statues). They're all in one area, so it's easy to see them. The Korean one, however, will show kids what real soldiers looked like, in a real war environment. It's an entire unit (life-size or bigger) going through a rice paddy, looking tired but wary, with backpacks, walkie-talkies and other real trappings. It's humbling to see the young faces on the statues. The rice paddy is suggested in one of the most creative ways I've ever seen.
This is way too long - just wanted to share some ideas. I love that city & have been there often on business. Anxious to go back!
Have a great trip, and come back with some Hubs to share!