Hi Emma,
Welcome to the Disney Parks Moms Panel! This is a great question and I think you are smart to plan ahead.
My boys are very early risers so we always keep a few food items in the room so no one has to move too quickly first thing. We like to bring light snacks, cereal, and bottled water with us and then purchase milk from either the
Disney Resort Hotel gift shop or the quick service restaurant at the hotel.
In my experience, I've been able to find quart-sized milk bottles
in the shops and smaller single serving sizes in the restaurants. Depending on your Disney Resort Hotel, you may have other options as well.
I hope this helps you plan many magical mornings at Walt Disney World, Emma! Wishing you a lovely vacation.
Andrea U.