Calcium deficiency and calcium supplementation seem to be a constant topic in pediatric clinics. It is well known that milk is rich in calcium, so why are there so many “little kids” who live on milk who are deficient in “calcium”? Many parents do not know that the process of calcium absorption and utilization by the body is controlled by vitamin D. Vitamin D deficiency can significantly limit calcium intake. Vitamin D deficiency can significantly limit the absorption and utilization of calcium. However, natural foods (including breast milk) are very low in vitamin D. The main source of vitamin D is produced by ultraviolet light from the sun when it hits the skin. However, with changing lifestyles, people have significantly less exposure to sunlight; air pollution, haze, and tall buildings, this has greatly reduced the opportunity to obtain vitamin D through sun exposure. Vitamin D deficiency is quite common in young children. That’s why the Chinese Medical Association’s Society of Pediatrics recommends starting vitamin D supplementation at 400 IU (international units) per day after 2 weeks of life until the child is 2 years old. It can be discontinued for 2 months during summer when there is more sunshine, if appropriate. Calcium supplementation depends on the situation. If the child is taking regular vitamin D supplements and has adequate milk in the diet, no additional calcium supplementation is needed. If the child has been deficient in vitamin D for a longer period of time and has developed skeletal changes, calcium supplementation is required.