We went in December last year and did the same thing. I made short sleeve shirts and we had jackets or sweatshirts for the early morning and evenings. You can always wear a long sleeve shirt under your t-shirt if needed. The late mornings and afternoons were beautiful and warm enough for a short sleeve t-shirt and shorts. I think layering is the key! I made short sleeve as I knew my family would get more use out of a short sleeve t-shirt after Disney then they would a long sleeve t-shirt.