Normally I wouldn't suggest an art museum for a second-grader, but the Dali Museum in St. Petersburg might be an exception because I know when I was that age, I would have found the optical illusions in his artwork to be really neat. The painting of a woman looking out a window that turns into a portrait of Abraham Lincoln as you move away from it was really something to see. I'm not normally all that interested in art museums even now in my 50s, but that one was an exception.
The Mote Marine Aquarium in Sarasota was interesting, and they have manatees. The nearby Ringling Estate would be less interesting for a second-grader except, maybe (and I emphasize "maybe") for the circus museum. I don't remember what admission costs, but I doubt it's worth it just to see the circus museum. The mansion is really the most interesting part of that place, but to see it properly you need to do the extra-cost tour that covers all the rooms and goes up to the roof, and I think that would probably be boring for a little kid.