Vitamin d (ergocalciferol-d2, cholecalciferol-d3, alfacalcidol) is a fat-soluble vitamin that helps your body absorb calcium and phosphorus. Having the right amount of vitamin d, calcium, and phosphorus is important for building and keeping strong bones. Vitamin d is used to treat and prevent bone disorders (such as rickets, osteomalacia). Vitamin d is made by the body when skin is exposed to sunlight. Sunscreen, protective clothing, limited exposure to sunlight, dark skin, and age may prevent getting enough vitamin d from the sun. If the baby feeding milk and taking nutrients it is not absolutely necessary to give calcium supplement. But there is no harm in giving to a growing child as his requirements of all vitamins are more. Let us say you can give till one year after that he must get all nutrition from good food. Happy parenting. Feel free to ask any doubt, privately.