I take my son to Pediactricare in Manassas. They have Saturday hours. Research the doctors at the practices you're considering. I checked reviews and credentials as well as weather or not any of the doctors have had any malpractice lawsuits. Good luck!
I'm with Kiaser out of Woodbridge and have been with them since I was a kid. I plan on taking my DD to them. Its a great facility with a good pediatric department. Unfortunately they don't offer routine appointments on Saturdays but they do have urgent care at the Woodbridge location should your LO need to be seen over the weekend or after normal hours. They also offer early morning appointments beginning at 7 am for some of the doctors. Hope this helps.
Close proximity was soo important to me, being in NOVA with all this traffic. We go to pediatric associates of Alexandria. We've been very happy there! Might be further north than you're looking for, though.
Re: Northern VA Mommys