To get the U.S. Recommended Daily Allowance for iron, which is 15 mg per day for infants less than a year of age, an infant would have to drink more than 31 quarts of milk per day. Milk actually causes the loss of blood from the intestinal tract, which, over time, can reduce the body's iron stores. Just how cows milk causes blood loss is not known, but researchers speculate that the culprit may be bovine albumin, eliciting an immune reaction which leads to blood loss. Pasteurization does not eliminate the problem. Some infants are exquisitely sensitive to cow milk and can lose large quantities of blood.
Milk does contain calcium.
But it is not unique in that respect. Kale, broccoli, and other green leafy vegetables contain calcium that is readily absorbed by the body. Broccoli actually contains more calcium per calorie than does milk. Beans are rich in calcium, and fortified orange juice supplies substantial amounts as well. There is no nutritional need for cow's milk in human children.