Another thing I think you have to consider is just how old you are considering old. Some grandparents are not real old. Are we talking 40s,50, 60s,70s,or older? Can they stand to be in the sun for long periods? Can they stand to be in the heat of Florida for hours at a time? do their meds for cholesterol/blood pressure or other type meds have warnings about being in the sun or heat?
There are some shady areas but not a lot of shade. Sometimes it is still almost unbearable in the shade if there is no breeze and I can attest it seems worse as you age. In my 40s and early 50s I was a summer camp director and outside everyday all summer but I could never do that now at 60. Sometimes I find being in a regular park all day in the summer very hard so tend to hit in the cooler times of the year more. I still find waterparks fun, but I swim and stay in the water. For someone who does not swim it might be rather boring unless they like to people watch.
I suggest dropping them off at SW and letting them tour SW at a relaxing pace where there is lots of air conditioned areas with chairs/benches while you all go to the water park, then meet them for supper and the evening shows at SW.