You can most likely swim in December. I think kids will swim in a heated pool no matter what the weather is. Like the others have said I've been hot in December at Disney and I remember a year when it froze and many of the flowers at the parks died (but I still think I took off my sweater by the middle of the afternoon).
we do like to stay out til park closing but like everyone has said, staying at RPR it's only a 6 min walk back to the resort to get changed into warmer clothes. We will be traveling with three teenagers so that part is easy. Guess we will hope for the best and hopefully get to use the pool midday. We are only an hour away from the beach at home and usually vacation mid summer so we are used to swimming daily at the beach or our Disney/universal vacays.