We've been to Disney in May and September. Both times my daughter and I wore sleeveless shirts or tank tops. She never once complained about being cold in the attractions. I think it's a combination of it being so hot outside, the cool is a welcome change and they are so absorbed in the attraction itself, they don't notice the temp.
I think they've complained about being cold twice, both times we got caught in a deluge on our way to the restaurant. They complained about being cold because they were wet in the A/C waiting for our table. Once the characters showed up though they totally forgot about it.
HTH,
Shelly