Hi!! I'm a Georgia native and I adore Atlanta. The Marta isn't that hard to use if you have used other mass transit in other cities(you will have to get a card at the station and only some have free parking) and driving in downtown Atl isn't that hard if you have driven in cities before, the interstate isn't to bad either but make sure you know your exit numbers and beware of toll roads. It's easy to take the Marta into the city, I've taken it to the High Museum of Art, it's also possible to take it to other attractions like the Zoo/Braves Stadium (same general location), and the Fox theater area, concert locations etc. Parking can be expensive so if you want to drive have cash for parking. Beware that there are a LOT of one way roads. There is plenty to do in Atlanta but you might have to do a little research first. I would recommend:
World of Coke (I loved it as a kid)
Atlanta Zoo (tickets are expensive, so beware but it's a great zoo)
Atlanta Aquarium (tickets are expensive I have heard good things about it but I prefer the Tennessee Aquarium in Chattanooga)
The High Museum is a fantastic art museum if your little ones like art. Plus it's easy to find discounts there. Parking here is VERY expensive.
Stone Mountain can be fun and it has a laser light show rash night (plus sledding in the winter).
The Fox Theatre sometimes shows movies during the day. The theatre is amazingly beautiful and an Atlanta gem, it is a Moroccan type theme and the theatre ceiling is a night sky with twinkling stars and clouds. They do plays and concerts as well.
Atlantic Station has a lot of shopping and I think it still has the Bodies exhibit. It is also where Cirque du Sole (not sure if I spelled that right) puts its tent up.
There are Civil War and Atlanta History museums galore.
In the winter there is ice skating in Olympic Park (also Olympic history)
Dave and Busters also has an Atlanta location.
Six Flags has an Atlanta location, look for ticket coupons.
Plus loads of other stuff.
Unfortunately things can be expensive in Atl, things like the City Pass and student/child/military/etc discounts can help. I've never stayed in Atl but I would stay downtown near a Marta station or expect to drive. Atlanta is not much of a walking city, things are spread out.
Oh oh!! Eat at the Varsity! The Atl location is the original, it's near the Fox and has greasy fast food and fantastic frozen orange drinks.