Not only is soy safe for children to eat or drink, it may actually 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.
The bottom line is that using soy milk on your daughter's cereal is safe and nutritious. Soy is a good source of omega-3 fatty acids, and is often fortified with calcium and vitamin D so you don't lose those nutrients if you give up milk.
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.