Hi Liz,
Welcome to planDisney! We're so glad you reached out to us with your question before your magical Walt Disney World vacation begins.
Lots of people don't realize that the Walt Disney World theme parks allow you to bring in your own food and non-alcoholic drinks. (No glass except for baby food or outside alcohol, please). Especially for families with young children, it can be a lifesaver to have snacks and cold juice boxes within easy reach.
Coolers or cooler backpacks up to 24" long x 15" wide x 18" high are allowed inside the parks. Loose ice and dry ice inside coolers are prohibited. While reusable ice packs are preferred, I've never had an issue with ice that is safely encased in plastic bags. I would recommend double- or triple-bagging to prevent leakage. Or I have frozen a couple of plastic water bottles to use as ice packs. Then, when they thaw, you have ice cold water to drink.
You can see a
full list of items that are not allowed inside the parks here.
Speaking of ice: Did you know, Liz, that all the quick service restaurants at the Walt Disney World theme parks will give you a free glass of ice water on request? Often, when it's hot, some of the busiest food windows will set out trays of pre-filled cups of ice water so that you don't even have to ask.
Thanks again for visiting planDisney! I hope this helps you make your plans. Please check back with us if you have any other questions.
Enjoy,
Wilma