If you happen to be in the area and looking for something to do with little kids on a cold and rainy day, or a very hot day, bring them to Dulles Town Center. This 2-level shopping mall has a nice indoor playground that is airport themed. Kids can slide down airplanes, steer the plane from the cockpit, crawl through the control tower, balance on the balance beam, and more. This is free!
The Food Court has a nice carousel that costs $1.50 to ride. There is also a Choo-Choo train on the lower level that costs $1.50 to ride (or you can buy a 10-pack of tickets for $1 each).
If you're in the Sterling area, check it out!