DC is fun, but there can be a LOT of walking involved. Many of the sights may also be lost on your 4 yo, but there are a lot of interactive exhibits aimed at younger kids and the 10 yo should love it. My kids' favorite is actually the Museum of the American Indian. We went when my girls were 5 and 7 and had a ball.

A lot to do outside the mall area, also. There is the zoo, and you could make a day trip out to Baltimore to see the aquarium and inner harbor and have some blue crab.