If your kids drink a lot of milk, it will cost you a small fortune. At the resorts you can usually buy a big carton of milk, which you can leave in your room if you have a fridge.
One thing we figured out was that it's often more cost-efficient to get your milk as part of a kids meal than by itself. For example, you can get 2 little mickey waffles, two pieces of meat, applesauce, and a milk for $4.39, whereas the milk alone is $1.50.