Milk is an amazing beverage. Human children routinely live on nothing but mother's milk for six months to a year, and many go as long as two years. In order for milk to sustain a child for two years, it has to be a perfect food. That is, it has to provide the child with every single nutrient the child needs in exactly the right proportions. Each mammal's milk is tuned to the needs of its children so that it is perfect for the species.
Milk can also provide all of the fluids a child needs.
In environments where the water quality is poor, the mother's body acts as a filter so the child receives germ-free fluids. Take human milk as an example. See How Food Works for a discussion of carbohydrates. See How fat Works for a discussion of fat. Human milk also contains the essential fatty acids a child needs.