Hi Lisa! Meal planning is just as important as anything else when you are budgeting for a vacation. Walt Disney World offers many different ways to dine while enjoying the parks and resorts but some families do like to bring their own foods with them. Sometimes this is because someone in the family has food allergies and it is easier to bring food then to try to find food that works.
The Disney parks do not have picnic areas for guests bringing food. That is the biggest issue as finding a place to sit and enjoy a picnic is not easy. Sitting on the ground or curb can work but it certainly is not as magical as a nice park somewhere.
Your second issue is how to bring the food in. Large wheeled coolers are not allowed. You can check out the size restrictions of all coolers
HERE. Bringing food in backpacks might mean a few squished sandwiches since you will have to keep the backpack with you unless you can find an open locker when you arrive.
If you bring your own food be sure and let security know that you have that in your backpack so they will take extra care when searching it.
Also remember that you can purchase some good healthy snacks while in the parks such as fruits. Sometimes knowing you can find this without having to bring it with you makes your day easier all around. Check your park map or ask a cast member to direct you to some of these locations. Whatever you decide to do I hope you will have a great day in the parks!!!!!