Not sure where you are from.... but I consider myself a Humidity Sauna like weather pro! I from the deep south, so I'm used to the wet soggy heat. Good thing about WDW is there is no mosquitos to add on top of it. (Thats like real Disney magic... its seriously amazes me)
Get you those neck fans... they look silly, but they help. If you have a stroller kid. Put more than one fan on them. I will be there again this month and I've bought her like 3 fans for her Stoller. She tends to get hot quickly and there is no air circulation in those strollers.
Cooling rags are also a must. We will take a cooler with us keep frozen bottles of water and keep our towels in there to stay cool. I have one that hangs on the stroller, and then I take a casserole carrier and use it as a cooler as well. Its insulated and slim to where i can fill it with frozen water bottles and keep in under the stroller and it doesn't get in the way. We hardly leave the parks for a break. Thats a personal preference and I know people thinks that's crazy. I kind of get anxious and feel like I'm wasting money if I'm at the resort during a park day.(but we do plan rest days) We plan our sit down meals during the hottest parts of the day to cool off. It helps!
DO TOYSTORY LAND AT NIGHT....... I REPEAT.... TOYSTORY LAND AT NIGHT.
its literally the surface of the sun during the day with no shade or escape from the sun. I feel like that's the biggest tip I can give you.
We take UV umbrellas. You can find some good compact size one
amazon to keep in your bag, and you can get one that can clip to a stroller or wheelchair/
Scooter. I have a cooler I use and attach to our stroller. Its really meant for the back of your seat in the car. But it works perfect for our stroller and gives us some extra cupholders for water bottles.. It also has a place for a tablet to go, but since we use it for the parks. I slide a piece of cardstock in there with our name on it. The best water bottles I have found to use are the 8 dollar
Walmart brand water bottles. They are slim and hold 24oz. They also keep ice a while and have a straw cover
We take the octopus stroller fans, Neck Fans, and I bought a fan that you can wear on your neck or clip on your waist band. Its amazing and blows really hard.
I've attached some amazon purchases below! hope they help!
Stroller Umbrella
Cooler
Waist Fan
Walmart Waterbottle