Soy is perfectly safe for children (and adults) to consume. In fact, it's been used as a replacement for cows' milk for many years. Some parents have expressed fears that isoflavones found in soy may have estrogenic (like the female hormone estrogen) effects on infant boys who are fed soy formulas.
Those fears are unfounded. The American Academy of Pediatrics states that soy formulas are safe and effective for infants and research shows no hormonal effects in long-term feeding of soy formulas.
Soy may be good for kids. Research indicates that soy beverages can have beneficial effects on cholesterol levels in kids who are overweight or obese. Consumption of soy beverages may also improve lung function in both children and adults who have asthma.
Giampietro PG, Bruno G, Furcolo G, Casati A, Brunetti E, Spadoni GL, Galli E. "Soy protein formulas in children: no hormonal effects in long-term feeding." J Pediatr Endocrinol Metab. 2004 Feb;17(2):191-6.
Smith LJ, Holbrook JT, Wise R, Blumenthal M, Dozor AJ, Mastronarde J, Williams L. "Dietary intake of soy genistein is associated with lung function in patients with asthma." J Asthma. 2004;41(8):833-43.