New York City: Train or Car to DC from NYC?
Washington, DC: Where to stay outside of the city?
Arlington (Virginia) and Alexandria (Virginia) are both great. Alexandria very picturesque, but pricier and further out. Alexandria, more sprawling, but less expensive. Could also try Roslyn.
Washington, DC: 9 year d boy
The National Air and Space Museum is pretty cool. The Museum of Natural History has LOTS of fun stuff for kids to touch, do, see.