Diet plays an important role in preventing adult cancer. By decreasing the number of cancer-associated food products and increasing the amount of cancer-preventing foods you eat, you may reduce your risk of some types of adult tumors.
Most studies about diet and cancer prevention involve adults who are exposed to dietary factors over long periods of time. The real benefit for a child may be in developing habits that will reduce the risk of cancer later in life.
Here are some great sites with information on nutrition and the prevention of cancer: