If you have ANY questions about the DC American Girl store, I am your person!
Not only did I live 3 miles away from Tysons Mall for 12 years, I worked at that AG store for several months when it first opened until I moved down here. I can tell you where to park, where to stay, where to eat, etc. I know that area and the store like the back of my hand. :-)
I've never been to the Atlanta AG store, but the Tyson's store is the largest AG store that is not a flagship store (Chicago, NY, and LA). I recently visited the Miami store, and I couldn't believe how small it was in comparison.
Coming from Clayton, it should be a 4 hour drive. You should have no problem getting there on a Friday evening. As you get closer to DC, you will see that most of the traffic is heading south on 95 or heading north on 95 above the city. We have been up there for the past 4 weekends getting our house ready for sale, and we haven't hit any traffic heading up. Saturday is bad in both directions as is Sunday. Best to go early in the morning if possible.
Weekend hotel rates in Tysons Corner are pretty reasonable. Forget during the week - it's very expensive, business rates - $150-$200 would be typical. You should be able to get a room for $79 or something at a Marriott on the weekend. I can give you specific recommendations if you want.
***Warning*** Traffic around Tysons Corner is crazy and congested. And they are doing work on the new Silver Line of the Metro which will open right across the street from the mall, so there is a lot of extra congestion there!
There are a ton of fun things to do around DC even if your kids don't like museums. If you are there for two days, you can do 1 day at the mall and one doing something else.
For the time at the mall, your 6-year old can go to the LEGO store if you think that will entertain him while you are at the AG store. There is also a play area on the third flood clear on the other side of the mall, of course, that he might enjoy. It's geared for younger kids, but he might like it. There is also a little train near the play area that does a little ride around the mall that he would probably like. There's a movie theatre, too, if he'd enjoy that. The average visit to an AG store is 2-hours, so I'm trying to figure out how you can entertain the others while your daughter chooses her doll.
Another option is to vist the mall in the evening. It is generally less crowded after dinner. During the summer weekends, it can be pretty busy.
If you are in DC and willing to drive into the city, a night drive while the monuments are lit up is spectacular. You don't even have to get out of the car. But just seeing the White House, the Capitol, the Washington Monument (although I think it has scaffolding on it) and the Lincoln Memorial lit up is very cool.
Your older kids might really like the spy museum. Although, it's one of the few that charge money and it's not cheap. But highly recommended! Air and Space is free downtown. And there's one out in Sterling Virginia near Dulles Airport. It's free but $12 to park.
There are a lot of local county and regional waterparks in addition to Splash Down, which is private and pretty expensive. There's one not far away from Tysons in Reston called The Water Mine which is fun, and much cheaper. And there are some others around, too. All of them will close Labor Day Weekend, though.
Hope this helps! Feel free to ask any me any AG question. My girls and I have been collecting the dolls for years.