According to the American Academy of Pediatrics (AAP):
The American Academy of Pediatrics recommends that infants be fed
breast milk or iron-fortified formula during the first 12 months of
life. Between ages 4 - 6 months, certain solid foods may be added.
Breast milk or iron-fortified formula, along with age-appropriate
solid foods and juices during the first year of life, provides more
balanced nutrition... (Source: Medline Plus)
Toddler formulas are offered as added nutrition for toddlers who are
picky eaters. To date, they have not been shown to be better than
whole milk and multivitamins. They are also expensive. (Source: Medline Plus)
Young children who drink cow’s milk increase their stores of vitamin
D, but decrease their iron levels. According to the study, “The
Relationship Between Cow’s Milk and Stores of Vitamin D and Iron in
Early Childhood,” in the January 2013 issue of Pediatrics (published
online Dec. 17, 2012), two cups of cow’s milk per day is sufficient to
maintain adequate vitamin D levels for most children, while also
maintaining sufficient iron stores. (Source: AAP)
There are disagreements about the fat content of the milk a child should drink (some say whole milk in small children because they need the fat source to fuel growth, some say that reduced fat milk is better past age 2 because saturated fats are not good for weight or artery health, and recent studies debunk the link between whole milk and obesity in children.)
There are also issues regarding the hormone content of milk and its effects on development in children:
It has been postulated that ingested estrogen in food derived from
sex-hormone-treated animals may play a role in earlier development of
puberty and increasing risk of breast cancer. However, no studies have
supported this hypothesis in humans. (Source: AAP)
And there is a movement afoot that suggests people should not drink cow's milk at all (they don't in many countries), but the NIH has found no evidence that it is bad for humans as long as there is no allergy, lactose-intolerance, or anemia (see the link for symptoms).