I love Disneyworld, and have been a few times. We went in July last time. It was very humid, and everyday there was a thunder storm, and for safety reasons when there is lightening in the area, some of the rides are temporarily stopped. The storms do nothing to freshen the temperature it stays humid, but hotels and restaurants have air conditioning blasting out.
If you are staying in Disneyworld the cheapest hotels, Pop century, All star resorts, are basic, but you are only in the rooms to sleep, they have good swimming pools and play areas. Staying in Disneyworld is a good idea if you can afford the hotels as it will be easy to return to your room for naps and rests when the heat gets too much. If you are going for the meal plan, just be aware that your 8 year old will have to pick from the kids menu, which will limit the food choices, but Disney is be no means all burgers and fries. Look on the Disney world website and you can look at menus. In fact you will want to make some advanced reservations, especially if you want to eat at specific restaurants such as Be our Guest, or Cinderellas castle. We liked to eat at the all you can eat buffet restaurants, Tusker House in Animal kingdom and Crystal Palace in the magic kingdom, because then you can really fill up if you need to. We have done the meal plan, and found we get plenty to eat, and do not need more than one snack a day, the only thing to bare in mind is that tipping is expected, and can add up over a fortnight. You can get free water from any counter service restaurant, so don't need to spend money on soda throughout the day, and at your hotel if you are doing the meal plan you get a refillable cup so can fill up on soda's at the hotel.
Buy some glowsticks, maybe even plastic ponchos from the pound shop before you go, and then they don't need to come out of your spending money. In fact I feel that once you have paid for the holiday package, which there is no denying is expensive, you don't have to spend loads there, especially if you can limit your DC's to one souvenir each. You don't have to get things like the photopass unless you want to, you can still go to meet and greet characters if you take your own photos, and you will have years of happy memories to look back on. Sorry this is really long, but hope you have a wonderful time.