The causes of most childhood cancers are not known. About 5 percent of all cancers in children are caused by an inherited mutation (a genetic mutation that can be passed from parents to their children).
Most cancers in children, like those in adults, are thought to develop as a result of mutations in genes that lead to uncontrolled cell growth and eventually cancer. In adults, these gene mutations reflect the cumulative effects of aging and long-term exposure to cancer-causing substances. However, identifying potential environmental causes of childhood cancer has been difficult, partly because cancer in children is rare and partly because it is difficult to determine what children might have been exposed to early in their development. More information about possible causes of cancer in children is available in the fact sheet, Cancer in Children and Adolescents.
Information Source: http://www.cancer.gov/types/childhood-cancers