Milk is pretty pricey at WDW, especially at resorts. It comes free with kids meals in the sit-down restaurants, but
otherwise, you can end up paying quite a bit for it. We always get a gallon on the way in and put it in our room in a cheap styrofoam cooler with ice. We only buy it in the parks when we have to, so that one-pint thermos usually goes along for the ride. I've discovered little freezable "ice pillows", they are little plastic freezer pouches about the size of ravioli; I put those in the milk inside the thermos to make sure it stays cool; though I do pre-condition the thermos with cold water and ice.
My 4yo will only drink milk and water; and he by far prefers milk. No juice, no kool-aid, no lemonade, no soft drinks.
We've gotten really good at the logistics of safe milk transport (sigh).