L.P.
I think Centenial Park is a must. Make sure to put the kids in a bathing suit. They have water fountains that come out of the ground which was a fun way to cool off on a hot day. Also not really in downtown Atlanta but is a short drive is Stone Mountain. They have a gondola that takes you to the top of the mountain and you can either walk down the mountain or take the gondola back down. At dark they have a laser light show to music projected on the side of the mountain. So bring a blanket and a picnic and have fun at a show. Also the kids are young but they have a kiddie place, there is always Six Flags. You can get to it using the MARTA and a bus. Hope you have a fun trip.