Living in Alabama, our summer weather can be just as brutal as Florida's. The comfort factor depends on how much rain we have had, which will boost the humidity into uncomfortable levels. Yearly weather patterns can change, so we may have an August where the temperatures stay in the low 90s, or some years it may be 105 or 108 during the day. Plan for the worst.
If I have to perform any outside activities during the summer, I try to prepare for it. Keep hydrated. Drink constantly, even when you don't feel like drinking. Wear light colored clothing - I like khaki, light blue, white or off white. I also try to perform my activities in the morning, when its cooler or after the sun goes down (there are times I mow my lawn in the dark!) I also avoid cotton fabrics next to my skin (socks and underwear.) Cotton may be light, but it fills up with your perspiration and will not dry out. After a while, wet cotton will act like sandpaper on your skin and you will have chafing.
During the summer, I wear moisture wicking socks and underwear. My shirts are also moisture wicking and loose fitting. I also try to wear synthetic pants or shorts if possible.
If I am going to be walking or exercising, I also apply stick deodorant to any areas of my skin that may be subject to chafing. I use Arm & Hammer deodorant, since it isn't alcohol based. You can buy Body Glide and products like that, but they are more expensive.
A wide brimmed, light colored hat is a must. Nothing worse than having the tops of your ears sunburned!
Have a good trip and enjoy Disney!