Contact Nysmith (in Herndon, VA). Seriously.
http://www.nysmith.com/
Nysmith is on one floor, with wide corridors, large rooms with plenty of room to move around, lots of "stations," etc. There is a small student/teacher ratio, and lots of individual attention.
In that area, there are also excellent public schools for your other daughter (and your younger daughter if you choose to move to public).
~ For ES, some of the great schools are Floris, Oak Hill, Lees Corner, Waples Mill, Navy, Greenbriar West, and a few others.
~ For MS, Franklin, Rocky Run, and Carson are fantastic.
~ For HS, Chantilly and Oakton are both very good, and Westfield is also good.