Stephanie:
You absolutely can take a cooler into the parks! There are a few guidelines, though, found
here under the frequently asked questions on the Disney website. Specifically, your cooler cannot be larger than 24 x 15 x 18 and may not have wheels.
You'll be asked to have your cooler checked before going through the ticket turnstiles, so also be aware that alcoholic beverages and glass containers will not be allowed inside your cooler. You can rent a locker for safe storage if you need to!
Using a cooler is a great way to save money on your food costs. If renting a locker cuts down on your savings, consider using smaller coolers (even soft-sided lunch boxes) in a backpack to keep your drinks and snacks cool. The backpack will be much more travel-friendly.
Wishing y'all a magical vacation!
Whitney