OK So Sunscreen every day while in the parks helps with preventing sunburn. Gold bond can prevent a rash, but also can soothe a rash. We mostly use it if we actually get a rash. Also cooling off in the AC will help with prevention. From what we've experienced and what we have heard most people that get heat rash, get it on their legs or core area. Gold bond worked for us. Don't worry so much about it unless you have to. Just enjoy yourself

Also be sure to take advantage of free ice water at most quick service restaurants through out Disney parks.