Anna!
Great question! So as it happens, yes, you are allowed to bring your own snacks and drinks into the Disney Resorts and the Disney theme parks. That is a great way to stay on budget, and ensure you have your favorite snacks on hand.
So I
answered a question related to this just recently. In particular, the guest was asking about using a small cooler in the parks, which is allowed. You do need to make sure that it fits within the specifications of 24" x 15" x 18". Also, do note that you cannot have loose ice or dry ice in the cooler. You can use ice packs, or bagged ice, but no loose ice.
I also recommend, if you are not going to have a stroller with you where you could leave your cooler, that you consider taking out a
locker rental. You will likely need the jumbo locker, if your bag is right at that largest size allowed, but you will be able to gauge which size is right for you when you arrive. That way you can store that cooler until you need it later in the day, making it easier to travel light through the park and on attractions.
Write me back if you have any other questions?
Have a Magical Vacation!