A lot of people love Rock Spring Children's Center, which is right by Rockledge. It's pricey, but has a great reputation. Getting a spot by September would be tough, though. Are you on any neighborhood listservs? That's where I tend to see the most recommendations for in-home places and various care options.
The term you want is center-based care.
Depending on where you end up in Bethesda, other terrific centers are Children in the Shoe and Wonders. Can't go wrong with either.