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. See How Food Works for a discussion of protein.
Human milk contains all of the different amino acids that a child needs in the right proportions.
Human milk contains all of the of the vitamins and minerals the child needs. The milks of different animals have different ratios. This page has a nice chart of many different mammals. It turns out that cow's milk is fairly close to human milk in terms of its percentages. Cow's milk has a little less fat, a little less sugar and about 3 times more protein than human milk does.