One thing that helps me know what to pack is the hour-by-hour forecast on Weather.com. I just checked today's temps (which would be similar to August). Low of 76 degrees, but that's at 4:00 in the morning! And the humidity is 93%, making it "feel like" 81 degrees in the middle of the night. By 2:00 pm, it's showing 88 degrees, but with the heat index/humidity, it feels like 99 degrees!
OP, back to your original question. I always bring a light, long-sleeved layer for air-conditioned restaurants. It's especially helpful if you get wet (either from a rainstorm or a ride like Kali!), since you'll feel even colder in the A/C. But lots of people don't feel chilly in the A/C, so it depends. I'd plan to bring one long-sleeved layering piece for each person - you may not need them, but if you do, you won't have to pony up the money to buy everyone something! WDW prices are expensive - one sweatshirt alone can easily be $50 or more.