Soy Milk and Protein and Children

Soy milk is nutritionally close to cow's milk, though most soy milk commercially available today is enriched with added vitamins such as vitamin B12. It naturally has about the same amount of protein (but not the same proteins) as cow milk. Natural soy milk contains little digestible calcium.
Soy milk is a good alternative for children over a year old who don't like or are allergic to cows' milk. Soy milk comes in different flavors, and it's perfectly safe to give those to your child. Soy is also a good source of protein.
Be sure to buy whole soy milk, not the low- or nonfat versions, because fat is important for brain development in children under 2 years old. Also, make sure the milk is fortified with vitamin A, vitamin D, and calcium.
You may want to make sure your child's diet contains other calcium-rich or calcium-fortified foods because soy milk contains phytates, naturally occurring substances found in whole-grain foods, legumes, and nuts that can decrease the absorption of calcium and other minerals. For example, while the label on a container of fortified soy milk may say that an 8-ounce glass contains 200 to 300 mg of calcium, the phytates can prevent your child from absorbing that full amount. Studies have found that the body absorbs only about 75 percent of the calcium from soy milk. Calcium-rich or fortified foods include broccoli, kale, lime-processed tortillas, yogurt, cheese, and calcium-fortified juices, cereals, waffles, and breakfast bars.
Because soy milk is plant-based, it doesn't have any vitamin B12, a vitamin that you get only from animal foods, including cows' milk. Pouring soy milk over a cereal fortified with vitamin B12 is enough to ensure that your child starts the day with the right amount of nutrients.
Many brands of soy milk highlight the fact that they contain isoflavones. Isoflavones are phytoestrogens, estrogen-like hormones found in plants such as whole grains, potatoes, dried beans, and apples that may lower blood cholesterol levels in adults. The phytoestrogens found in soy milk are safe for children and adults.

http://www.kidshealth.org/parent/medical/heart/ida.html
Frank M. Sacks MD, et a. (2006) Soy Protein, Isoflavones, and Cardiovascular Health. An American Heart Association Science Advisory
http://www.thedietchannel.com/Soy-Milk-Healthy-For-Children.htm