Bring at least a little warm stuff. I was at WDW October 8-20, and the first several days were 90+ each day, high humidity. Wore sleeveless tops, lightweight shorts and sandals, and didn't even need the sweater I had in my backpack when indoors.
Then, over the course of three days, temps dropped more than 30 degrees! I only had one pair of long sweatpants, no warm socks, and one lightweight jacket. It was so windy at Epcot, I was so cold! What I wouldn't have given for a warm jacket, long socks, etc.! Wouldn't have taken up much room in my luggage, but I didn't even think about it! Wasn't willing to pay more than $50 for a WDW sweatshirt or jacket, and simply couldn't find long socks to buy anywhere! (I did end up finding a pair of mid-calf socks at Japan pavilion, which helped some, but I was cold!!) I wish I'd had gloves!!
So my advice - bring at least one pair of long pants, long socks, and a warm jacket for everyone. You never know what the weather will be like!