Atlanta has a three great museums within a stone's throw of each other.  They are the Michael C. Carlos Museum at Emory University, the Fernbank Museum of Natural History on Clifton road, and the smallest of the three, the Fernbank Science Center, a perfect place to take the kids for a great museum trip and a wonderful hike through an old-growth forest at a price anyone can afford.  I'm recommending the Science Center for kids under 10 because the Carlos Museum focuses on the art of the ancient world, which can go over the heads of the smaller kids (although it's voluntary contribution entry price is an amazing bargain), while the Fernbank Museum costs an arm and a leg and takes all day to tour.  The Science Center possesses a Tyrannosaurus Rex skull, a planetarium, and some genuine moon rocks near a reentry-scorched Apollo spacecraft - it is a small place that deftly covers all the bases for excitable kids.  The main hall is also free!  You may make a donation when you enter - planetarium shows do charge a measly $4.00!  I love the forest that stretches for 65 acres out behind the Main Facility.  The ancient oaks tower over a long valley that really can feel otherworldly on a nice day.  Check out the website of the Center for any questions you may have!