I just want to reassure you. I was there Sept 14th to 23rd. Our first park day was the 15th. It was hot, very hot and more humid than I am used to.!
Now I do not like high heat. I would much rather be too cold than too hot. It was not as bad as I had thought it would be! Drink lots of water, use and reapply water/sweat resistant sunscreen, wear a hat, do some inside rides/shows to get out of the heat, splurge on some frozen lemonades ;-), get a misting fan, take an afternoon swim break. I actually wore a knee length full skirt one day! It was way cooler than capris or shorts for me!
Have fun! We had a blast! We laugh now at home some of the silly things we did because of the heat. We had a squirt fight with the mister. I, who does not like getting wet while in clothes, actually went on Splash many times and did Grizzly!! In cooler weather I would have sat out.... I had fun with my family on those rides! We went on the Ariel ride when coming off the boardwalk just to cool down. Out kids are a bit old for the Ariel ride.
No matter what you are at DL!!!! You will have fun regardless!!!!