Hi Amanda, and welcome to the Disney Parks Moms Panel!
You absolutely can bring your own food into the Walt Disney World Theme Parks - in fact, I think it's a great idea! It's a perfect way to help control costs if you're on a budget, and while there are plenty of healthy options on the menus, you can often make healthier choices if you've pre-planned your meals with food you've brought along.
There's a handy list of things you can't bring into the parks on the Disney website. Other than glass containers, there isn't much that applies to food. You can even bring in a soft-sided cooler as long as it's smaller than 24" long by 15" wide by 18" tall.
And here's a mom's tip for you. When you're ready to dig into your food stash at the Magic Kingdom Park, head over to Storybook Circus in the Fantasyland section of the park. Walk past Dumbo the Flying Elephant, head left when you get to the Casey Jr. Splash 'N' Soak Station, and look for some umbrella tables to the far left of the train station. It's a shady spot to enjoy your meal, and the kids can watch the train pull into the station. It's one of my favorite places to relax for a few minutes. And not many people even know it's there. Shhh!
I hope this helps you enjoy time at Walt Disney World, Amanda. Please stop by and see us again if you have any more questions. Wishing you a magical vacation!
Kathy K.