Mommy, weather in Florida in early-mid December is relatively mild. Temps are usually in the mid 60s to 70s during the day, down to maybe the 50s at night. Of course if a front makes it's way down, at worst it might be in the 50s during the day and 40s at night. Probably not much in the way of rain.
Crowds shouldn't be too heavy. You probably know the drill - get to the parks early, use fast pass etc. Crowds will definitely build as the holidays get closer.
Overall, a good time to go. I think as time has passed, more folks are figuring out it's a good time to go, so it's not as slow as it used to be. But still, it's a lot better than the week between Christmas and New Years and less crowded than the summer months.