Being from a more temperate state (Maryland), I find the average temps in WDW during early December to be usually too cool for shorts. I have seen temps in the 70s, but below 70 is too cool for me to wear shorts.
However, on many days it will get plenty warm enough for t-shirts. On a few days it might even get warm enough for shorts, but the night temps go down enough to make them uncomfortable so I rarely wear them in early December.
Pack several pair of long pants, 1 pair of shorts, all short-sleeve shirts, and a couple of sweatshirts. Check the 7-day forcast the day before you leave to see what the high and low temps will be, and if they are expected to be warm enough, you can add some more shorts to the packing list.