Good luck actually bringing the food in. I have lived in DC for 8 years now and we go to the museums a lot. I have had packaged snacks and drinks in my diaper bag for the kids and had to throw them out. We learned the lesson after the 2nd time and have stopped bringing the food in and with the kids being older now it is not a problem. They won't let you out of the food courts with food either, which I know as I was sent back into the food court at Natural History with the bottle of water that I purchased there.
Of course, it is possible that they will miss a drink or snack from time to time, but I would definitely plan on that not being the case and being prepared.
The Air and Space McDonald's is pricier than most, as are the Starbucks in prime locations in the city. Or anywhere in the city compared to other places for that matter. They can set their prices to be whatever they want. Of course, being a McDonald's, it is still cheaper than other places, but more than you are likely used to. The food in the immediate vicinity of the museums and monuments is also pricey, so not really like you can leave the museums and get some cheap food nearby. We don't have a ton of fast food eateries in the downtown area (like Wendy's or whatever), so mostly sit down places.
Regarding the airport, are you flying Southwest? If so, their terminal has a food court, diner, and Phillips crab restaurant. If you are on any other airline the food choices are minimal, unfortunately. The only thing is, you'd have to leave your bags spinning on the carousel cause I think there is just 1 restaurant outside security (and it really isn't that good).
But the going against traffic thing, unfortunately there is no for or against traffic when it comes to the DC area. My friend is currently driving to NJ from DC and just called me to say that it has been over an hour and she isn't even halfway to Baltimore and it's noon on a Wednesday! We fly out of BWI frequently (though each time we say it's the last cause we hate the drive!) and it can be a big drive if there is any traffic.
Anyway, like I said, I live inside the city, just a couple of blocks from the zoo.
I completely agree with what all the other locals are saying and, honestly, it just seems like you think we are all crazy, making up random stories, or trying to dissuade you for our own benefit. We have friends who live in Vienna, near the Dunn Loring stop you'd be taking, and it is far. I am not saying I wouldn't stay there no matter what, but it is a big commute. Our friend takes the Metro to work and he is on it a long time (and he works at at the Foggy Bottom stop, so the first stop inside the city on the orange line), plus in the mornings the trains are always standing room only when he is riding them. While that is a pain when it comes to commuting, on a vacation with 3 kids, well, I have 3 kids and it is not something I would want to do.
BUT, it is not my vacation, so if you don't mind riding packed trains, never going back to your hotel for a break (and July here is HOT and so humid!), and getting up really early to eat before getting on the train (cause you can't bring any food or drink on the Metro, they will fine you, which I know cause my husband is the recipient of one such fine, and he was not consuming his drink, just holding it on the train), then I am sure you will be fine.[/QUO