Calcium is an important mineral for building bones. Young children are growing so rapidly that they need a steady supply. Milk, however, it is not the only calcium rich food.
There are two issues with relying on milk for calcium. First of all, many kids react poorly to milk; pasteurized milk is one of the top allergens. Secondly, many kids over drink milk and get too much calcium and phosphorus. Excess calcium can be a problem as it inhibits the absorption of some other important nutrients such as zinc and iron.
By drinking too much milk, a child's immune system can become weakened due to insufficient zinc and iron. For this reason it is good to become familiar with a variety of calcium sources and lessen your reliance on milk.
Breastmilk is rich in calcium so it becomes more of an issue only after weaning. See below for a list of non-dairy calcium sources.
It's best to include as many sources as you can rather than relying on only a few.