Mim,
You say that you "plan on going to GMU". Since you have kids, I'll assume that you are going to graduate or law school there. Mason has 3 major campuses: Fairfax, Manassas, and the law center in Arlington. I would think your choice should probably be influenced at least in part by where you will spend most of your time.
Will you be in school full or part-time? If full-time, you should start focusing now on making sure your signficant other makes enough money to pay for your living expenses. Even if you become a TA, you won't earn anything near what you'll need to live here. Buying a house would clearly be out of the quesiton, unless you are independently wealthy -- in which case why worry. You probably shouldn't be thinking about buying anyway, because you don't know where your future lies. Say, for example, you're getting a PhD in Economics. When you finally finish your oral defense, that tenure track teaching job you want might be in Texas or Indiana. Any one school doesn't have that much churn in faculty, so you'll most likely re-locate. Why be stuck with a house to sell, even if it doesn't take very long to sell one here.
How old are your kids? If little Skippy and Hoppy are elementary school age, uprooting them is pretty easy. If Junior is a sullen sophmore, you'll have to look more carefully for a good match. You must not be from RoVA (Rest of Virginia, as opposed to NoVA, Northern Virginia), because you would know about the reputation of NoVA schools. Truth be told, Fairfax County's public school system, FCPS, is very, very good, though not quite as good as county officials and realtors would have you believe. It skates on its reputation a bit. In my view, Arlington County Public Schools is every bit the equal of Fairfax. My view here is based on my having gone to and graduated from FCPS, my kids are currently in the FCPS system, and I have worked for Arlington.
The City of Fairfax is independent of Fairfax County (Virginia is unique in how this works), but the schools are run by the County. The city of Falls Church, on the other hand, is also independent of the surrounding counties (Fairfax and Arlington), but runs its own 1900 student system. Falls Church is roughly equidistant between the Fairfax and Arlington campuses, so you might just end up there.
Academically, as you know, a school's quality is really determined by its service territory and the effectiveness of its Principal. Parents in this area are well-educated (highest percentage of Bachelors degrees in the nation) and demanding. The quality of the schools reflect that, which attracts teaching talent. But this isn't Heaven; it isn't even Iowa. Most of the teachers and administrators are good, dedicated folks; but you'll always find pockets of incompetence protected by indifferent bureaucracy. You'll be happy with the overall quality of your kids' education anywhere in Fairfax or Arlington, but depending on their needs, you may have to be proactive in assuring they are getting it.
The region is highly diverse, with demographics ranging from kids on subsidized breakfasts and lunches to kids on subsidized payments for their Cadillac Escalades. There are ethnic enclaves here, as in any big city, including WASP in McLean/Langley/Great Falls. You may like this, you may not. You may be a member of one of them. You are the best determinant for what is a "bad" area. If you ask a question like that on a forum like this, the responses will vary from the silly to the considered to the down right bigoted. Come here, talk to your peers at Mason, and decide yourself. This is hardly an armed camp, but crime happens everywhere.
What you should really do first is check with Mason's Office of Off Campus Housing. They have staff and resources just for people like you. Their website is
http://och.gmu.edu. (I don't work for Mason but another institution of higher education in the DC area).
I've been in your very situation. Kids are malleable. It's an adventure. Good luck in your studies.
Oh yea, one more thing. What everyone says about the traffic...it's all true.