Hi there,
We went to Disney back in 2008 and like others have said the weather is changeable. I remember having to wear sweatshirts first thing in the morning and could sometimes see my breath, This didn't last long however as by around 10.00am it would be shorts and t-shirt weather. Looking back at photos my 2 daughters were often in suntops and shorts or summer dresses.
Evenings would usually be jeans and sweatshirts/t-shirts. In fact very much like a british summer, although it didn't rain once so maybe not! We had booked to go to Mickeys Christmas party one evening and we all set off in jeans, sweatshirts etc and were roasting even at midnight, people were still in sun dresses.
A few mornings we also had to clear frost off the car! something you don't expect in Florida!
All this said however it was a fantastic time to go and I enjoyed it far more than our usual humid trips in September. The crowds are very low (we went from Nov 29th to December 15th) but you get to experience Disney at Christmas which is magical.
As previosly mentioned beware of Pop Warner. We stayed at the Riverside on that particular trip and to be honest they spoilt our stay somewhat. The parents were just as bad and thought it was perfectly acceptable to impose themselves on everyone else. The foodcourt was a complete and utter waste of time at breakfast and certain times later on in the day as they would all visit en mass before or after tournaments, unless you wanted to waste an hour of your day standing in line then it wasn't worth bothering with.
The place was overrun by precocious , young girls each wanting to be seen and heard more than the next one and none of them showed any regard for the other guests... it was 'All about them'!! Our children were often woken late into the evening by the screaming girls and their parents. I wish now we had changed resorts, an error on our part.
My advice would be to book up, I would in a flash, as long as you take layers like others have said and check first that Pop Warner girls aren't staying at the hotel you're interested in the you can't fail to have a fab time!
Happy planning,
Jane.