I've never travelled with a little one, but I'm an experience Disney traveller who almost always goes at this time of year and can tell you from experience it is best to pack a bit of everything. So the same should hold true for packing for a little one as well. You should pack a little bit of everything for all kinds of weather as you never know what kind of weather you will get. The weather can change down there from one day to the next.
One year it was 85 and sunny every day and we were in t-shirts and shorts at midnight in the Magic Kingdom. Last year was all over the place. Some days we got away with t-shirts and shorts in the day and jeans and sweatshirts in the evenings, other days we were getting up and it was high 30s/low 40s in the mornings and we were lucky to crack the low 60s during the day.
Also it never hurts to pack a pair of mitts/gloves, especially for a little one, especially if you are staying at a resort like the Wilderness Lodge where you will be travelling back to the resort at night over open water. What most people don't realize is that the temperature drops quite dramatically and quickly once you are out on the open water and away from land.