I didn't see anyone mention this above, but the pools themselves are heated to about 80 degrees.
I swam with my two young sons (my wife declined) when the air temperature was around 55 degrees in the afternoon, last November 18. There was steam rising out of the water, due to the temperature difference with the air. I was actually more comfortable in that pool than in my local _indoor_ pool, which is heated to 78 degrees. I was the only adult in the pool, but there were a few kids besides mine. The boys would have swam all day if I let them. When we got out of the pool, we bundled up in towels, walked quickly to our room, and immediately jumped in the shower. Their lips were purple by the time we made it to our room, but it was really fun. They both fell asleep for a 90-minute nap right after the shower, and we still had time to go back to an empty Animal Kingdom around 5 pm for dinner and a few rides.
That was the coldest day of our trip. Many other days were sunny with highs in the low 80s, with lots of people in and around the hotel pool.