Plant estrogens, also called phytoestrogens, are natural compounds found in many foods. There are two main groups: the isoflavones and the lignans. Isoflavones, which include genistein, are found in soy beans and are the most widely studied of the phytoestrogens. Lignans are found in flaxseed, cereals, fruits, and berries. Phytoestrogens are strikingly similar in chemical structure to the common estrogen estradiol and and can mimic many of the effects of the natural hormone. Most research on health effects of phytoestrogens, including effects on breast cancer risk, has been done on soy products and genistein.
Most (but not all) studies suggest that regular intake of soy, as a well-integrated component of a regular diet (as opposed to a dietary supplement like a pill), may be slightly protective against breast cancer. On the other hand, studies that look at regular consumption of soy during adolescence, again as part of a regular diet, provide significant protection against later breast cancer development. Some studies examining the effects of ingesting high levels of soy supplements (e.g., genistein pills or isoflavone protein extracts) suggest that this may lead to changes that increase the risk of breast cancer.
So eat a healthy, balanced diet that is rich in vegetables (preferably locally grown and pesticide-free!) including soy products. Introduce your children to soy products (soy flour, tofu, etc.) early in their development, as part of their regular diet. But stay away from concentrated or isolated forms of soy derivatives, including genistein pills.