Regarding clothes.
The clothes that are supposed to wick sweat have not kept me cool and dry. The clothes I have tried have been made out of polyester which is hot. The clothes do dry fast but they make me sweat more than clothes made of good old cotton. I want to be cool and dry. Not wet and cool.
The shirts that wick I now wear in the winter because they are so much warmer than a regular T shirt.
I would try these types of clothes at home before going to WDW.
My grandmother who lived all of her life in south GA and in FLA always had light weaved cotton clothes to wear in the summer. She knew what to wear to stay as cool as possible.
The idea of keeping something cool on your neck is very good advice. Your neck and head radiates quite a bit of body heat. If the sun is beating down on your neck and head you will over heat quickly. Farmers, field hands, ranchers, etc, wear wide brimmed hats is to keep that sun from directly hitting their face, head and neck. A wide brimmed hat makes a huge difference but you need a hat the breaths yet shades your head.
If I have to work in hot weather I wear a wide brimmed hat and put a small towel under the hat and down my neck. Think of the French Foreign Legion Kepi. I wet the towel from time to time to keep it wet. The water evaporates and cools you down. It works. It is easy. It is cheap. Might look weird though.
Do not forget to eat as well as drink. If you sweat a lot, you are loosing minerals. You MUST replace those minerals or you will have problems. Just drinking water will not help and makes it worse. When it is really hot, eating is not really what one wants to do, but you need the energy and minerals. Small snacks here and there help. When I work in the heat I eat dried pineapple, bananas, apples and unsalted peanuts. I just grab a handful along with water when taking breaks. It helps keep you going.
I would be careful of products that stops or reduce sweating. I had a very bad experience with bug spray many years ago. I worked on a horse ranch in FLA. One morning it was already hot and humid and then it started raining. We had to work on a fence line so we put on some rain coats and bug spray because the squeeters where really swarming. There really was no reason to wear the rain coats since all they did was over heats us and made us wet with sweat. Would have been more comfortable to just work in the rain.
At lunch I was very hot. I took a long drink of very cold soda and promptly had extreme pain in my stomach, seizures, and I passed out. Scared my coworkers just a bit.

I came too pretty quickly and we tried to figure out what happened. I read the bug spray can which was from a major company. On the can it said that the spray prevented sweating and might cause you problems in the heat. DUH!

I have read bug spray cans ever since and no longer see the warning but I still read the cans.

Not sweating could kill someone from over heating.
Later,
Dan