We've been to Disney in early December twice in the last 3 years.
2011 was COLD - the sun made the days relatively comfortable in jeans and a long-sleeved top (or t-shirt layered under a sweater), but we were bundled up in fleeces and jeans and scarves and gloves every evening. And still shivering sometimes. I regretted leaving my winter coat at home! My son (who was 2 1/2 at the time) wore a fleece during the day, and his winter coat with the hood up over his head at night. We did the Very Merry Christmas Party and had to buy my son a warm blanket to tuck around him in the stroller, because he was simply too cold, even in his coat!
2012 was much warmer - daytime, we were comfy in jeans/capris and a t-shirt and even got pretty sweaty in the sun a few days. We were able to go swimming at our resort, though it was a bit cold once we stepped out of the water! Evening temps were cool, but not truly cold. It was far more pleasant that way!
We also did a one-day stop one Christmas, several years ago. That year, it was colder in Orlando than it was back home in New York. I remember walking around EPCOT with a totally numb nose. Crazy.
I'd recommend looking carefully at the weather forecast for Orlando before you pack - both the high temps and lows. Bring along at least one warm fleece or jacket + scarf/gloves, just in case, especially if you plan on being out and about in the evenings. In general, layers will be your friends! I think you're on target with jeans/yoga pants and tee's + cardis during the day.
Meanwhile, I'm hoping for a relatively cool Halloween at Disney World. I'm hoping that we're not dragging around the parks in 85+ degree weather and full humidity at the end of October. I wouldn't mind a little "perfect" weather on our trip, too!
