Hey there, hi there, Lisha! We're so glad you came to us with your question.
There are some restrictions when visiting the
theme parks and other areas of
Walt Disney World. We'll start with the good news, which is that you are most welcome to
bring your own food and beverages to the theme parks with you. You can't bring glass containers, except for baby food, and there are a few other things to be mindful of, such as they can't be foods that have to be reheated or have a pungent odor. But with few exceptions, you're good to go.
Where we hit a snag is the size of the cooler. The
size restriction is not based on volume, but on the size of the cooler itself. Whether or not the cooler is on wheels or backpack style, you are limited to one that is at most 24" long x 15" wide x 18" in height. The restrictions are designed for the safety and well-being of guests, so they are pretty strict when it comes to making sure your bag meets the measurement restrictions.
I used to have an Igloo backpack cooler when my girls were younger, and I see that they still do make bags that meet the size limitations. I'd look into other options so that you won't have to worry about not being able to bring it in when you get to the parks.
I wish I had better news, but I hope there's enough time to replace the backpack you have!
Anna