Weather Underground says to expect highs around 80 and lows around 60. My uncle lives about an hour from Orlando, and says that is accurate.
I'm planning on shorts for the kids and shorts or capris for me, short sleeves, and a sweatshirt for everyone for the early AM and evenings. I think we'll skip the water parks, but if we get wet on Splash Mountain or Kali River Rapids, I don't think we'll freeze!
I've been there in January and February before, and never needed more than a sweatshirt or light jacket.