Hi Julia!
I've actually visited Walt Disney World under very similar circumstances as yours. I found keeping a few things in mind helped to keep the trip a great one.
The heat is a considerable factor, especially with little ones. I probably don't have to tell you how important sunscreen and staying hydrated is, but I would like to remind you that if you need a cup of ice or ice water, most locations selling beverages will be happy to provide these. You may bring in snacks and beverages into the parks if you wish, too.
Escape the heat when you need a rest. Dining locations can be helpful in this regard, since you'll be indoors and refueling. Also, baby care centers (you can find them on the maps at each park) are a great spot to change, feed, and even just cool down your little one.
It's also just as important to remember to go at a pace that's right for you and your little ones. Sometimes, this can be easily forgotten when traveling with extended family or larger groups. But I think kids need to enjoy the Disney parks at a different pace then the rest of the family. When I travel with my extended family, I always remind them, as much as I'd like to spend all our time together, my kids would be better company if we sometimes took breaks and then met up again at a later time.
One more thing I'd like to share with you. If you go
here, you'll notice helpful information about traveling with little ones. In particular, I like the pdfs with attraction height requirements and park maps highlighting attractions geared towards little ones. These are great for planning your time in the parks. I hope you find there are plenty of attractions for your little ones to enjoy!
Come back to the Moms Panel if you need more help in your planning. Have a magical vacation!