There is a Ritz-Carlton hotel connected to the Pentagon City shopping mall -- may well be more functional to be proximate to a mall with 3 kids. this mall is on top of a subway station and not far from the national mall, though it is in Virginia.
That's a lot of time in museums for 3 kids. Non-museum ideas: baseball game at Nationals Park. Horseback riding in horsey country a la Middleburg, VA area (a day in the country -- would need to rent a car for a day). Hiking at Great Falls (Maryland or Virginia). Mt. Vernon. Hopefully you'll have a day or two for which the weather will be less hot and humid.