I can't speak for the crowds, but I used to live in Central Florida, so I can definitely talk about the heat. Air temps for Orlando/Kissimmee will easily be in the mid to upper 90's. Add in concrete and sun and no shade, and it can be 100 degrees in the parks. It won't get to 110 like Arizona, but you do have to factor in the humidity. You will sweat, and it will not evaporate, so it won't cool you. But you can work with and around the heat.
The standard "Go early, leave in the afternoon, and go back in the evening" plan does work. My family refuses to do it, however. So my workarounds are:
1) Dress for it. Cooling/wicking clothes, light cotton, dresses and skirts and shorts. HATS. Sandals if possible. Light colored clothing is better.
2) Take any chance to get in air conditioning. The shops are air conditioned. Indoor shows and rides, rides with indoor lines - they all help. The restaurants are air conditioned - we eat counter service exclusively, but even going inside long enough to grab condiments and toppings can help. Sit in the shade - as long as you're in the shade, it's better.
3) Ice water. Drink a lot of water, enough that you're visiting the bathrooms. That means you're staying hydrated, and the bathrooms are air conditioned! Splash some water on your face and neck while you're in there.
4) Cold snacks. And then sit down in the shade to enjoy them.
5) Water rides, walk through splash pads, splash some water on your face.
We generally do a ride or three, then take a break with a snack, or a meal, or a longer indoor ride or show. Carousel of Progress is a great 20 minute chance to get air conditioning, and we also all like the show, even the kids! The line for Pirates of the Caribbean is almost entirely air conditioned, as is the line for Dinosaur!. All of the movies at EPCOT are air conditioned.
We also take family walks for several months before the trip, to help acclimate to the walking and the heat. Not sure how much it helps, but it doesn't hurt!