Calcium is completely essential for good growth and development but it is possible to have too much of a good thing. A child should not exceed 2,500 mg of calcium per day either through food, drink or supplements (or a combination of these) as anything above this is considered unsafe.
An excess of calcium can cause iron deficiency as it blocks the absorption of iron, as well as other minerals such as zinc, magnesium, and phosphorus. Consuming large amounts of milk can also add extra calories that can contribute to obesity. And kids who fill up on milk often lose their appetite for other healthy foods.