Hi Jessica,
Welcome to the Moms Panel! Having just returned from an unusually warm June visit to Walt Disney World, I can tell you the heat is certainly something to take into account when planning your days.
Leaving the parks in the late morning/early afternoon was key to the attitudes and endurance of my family. Having a nice long swim in the afternoon, heading to Downtown Disney for some shopping, or simply a nice rest back at the resort are great ways to avoid the mid-day heat and crowds at the parks. I found that once the sun started setting, my kids were recharged and I didn't have to worry about the sunscreen, this made for some very pleasant evenings!
Any of the attractions that were open during the day, will remain open for the duration of the park hours (except Extra Magic Hours when only certain attractions are available). So spend an evening at the park, I'm guessing you'll find it quite enjoyable!
For more information on traveling to Walt Disney World with little ones, check out the in-depth planning information
HERE.