How old? But I don't even think that moves the needle.
Key West, hands down. The rooms are quite nice enough, and (I) just think of it as a base. Son was completely content with the TV when we were in the room. The pool is very tame - small enough that it can't be otherwise - and you could watch the kids - depending on their age - right from many rooms. But biggest, IMO, are that Mallory Square, and Duval St. for that matter, are just steps away. Constant free entertainment. Also coming and going cruise ships are very impressive, up close. And just mention the green flash and that'll work on their minds until it becomes a quest.
As to Atlanta, there's some neat stuff, but I don't think enough for it to be that great for kids. (Coda back to TV comment above, as you likely will be doing in the event. In fact, if you're at the conference much of the time, it's the TV for them, likely.) If you're comparing historical stuff, President Truman's Key West winter white house is in walking distance.
Again, age matters to the consideration, but I don't think even that would move the needle.
Almost forgot - if they're young, the turtle pond in the courtyard is appealing to young kids.