I ditto the Loew's L'Enfant Plaza suggestion. They have a rooftop pool and is steps from the Smithsonian and the mall. Also I would add the Holiday Inn at the Smithsonian, I believe on Ave. C. They also have a rooftop pool and is located directly behind the Air and Space Museum. In DC you'll be walking a lot in the heat so a central, easy location to return to with kids would be a good idea. I've stayed at both hotels. Both I would recommend and stay again, primarily because of their great locations. Enjoy this great city!

Viv555, let us know what you do and how you liked the place you stayed!