I'm an adult who wilts quickly, so while this is from my POV, most of it would also apply to a kid.
If you can go off-season in January or February, you'll have more choice of when you go. It's also cooler so if heat is a trigger for worse problems, it won't be as much of a concern. The drawback is that anything water-related is usually colder (and sometimes they close one or both of the water parks to do repairs during the winter. When I've been in late January, there are very few long waits so it doesn't matter so much if you get there later.
If you need to go during the summer or one of the busy times, it's definitely worth considering at least trying to make rope-drop. If heat makes her wilt faster, you probably don't want to have her in a park during the afternoon when it's hottest. That break is a good time to take a nap (chances are everyone will be tired enough for one at that point) or do other things around your resort. You can go back in the evenings when it cools down, if you'd like, but I know that's harder with a 5 year old.
Whenever you go, make sure you use your FP+ for any must-do attractions, especially those that tend to have longer waits. If you're staying on property you'll be able to schedule in advance. If you're staying offsite (I'm assuming you don't have annual passes), you'll be able to schedule yours when you first get to the park. Then you can fill in around those with activities (attractions or other) that don't have long waits.
You may want to consider getting a stroller as a wheelchair tag for her. That will allow you to bring her stroller into lines for rides and other places that strollers aren't usually allowed. Most of the stand-by lines at WDW are accessible and those that aren't, there are other ways to access the attraction using a wheelchair/mobility aid (the one exception is Tomorrowland Transit Authority which requires riders to be able to ride up a moving walkway on an angle).
If needed, she will be able to lie down in First Aid for a while. Sit-down indoor meals or air conditioned shows can also be useful if avoiding heat will help her last longer.