When we're young, bones grow in size and density, fueled in part by calcium. When we stop growing in height as teenagers, our bones continue to grow more dense until about age 30, according to the National Osteoporosis Foundation. After that, bone mass and density can remain the same, or bone loss may begin to occur at the rate of about 1 percent a year. That's why calcium is crucial during childhood.

The RDA for calcium is as follows: Kids ages 1 to 3 need 500 mg a day; those ages 4 to 8 need 800 mg; and those 9 to 13 need 1,200 mg daily. One cup of low-fat milk contains 300 mg of calcium. Not all children, however, can stomach dairy products, which are a traditional source of calcium. For those who are lactose intolerant or allergic to dairy, good nondairy sources of calcium include calcium-fortified soy milk and orange juice, as well as broccoli, tahini, pinto beans, kale, sea vegetables, tofu processed with calcium, corn tortillas processed with the mineral lime, almonds and calcium supplements.