1 IF A CHILD GETS EVEN ONE BLISTERING SUNBURN, HER RISK OF MELANOMA DOUBLES. About 80 percent of the most dangerous lifetime cancer-causing sun damage occurs before age 18, so it’s crucial to protect your kids now. Sun exposure during childhood changes the skin’s genes or DNA permanently, causing skin cells to stop functioning properly. “And years of repeated sun exposure will harm skin cells further,” says David Leffell. M.D., a professor of dermatology and surgery at Yale University. That means that as your child ages, she’ll have a higher risk of basal-cell cancer, squamous-cell cancer, or melanoma (the least common but deadliest form). “But If you can block that first hit to the DNA, you may be able to prevent skin cancer from developing later on,” Dr. Leffell says.