Tooth decay and cavities are some of the most common chronic childhood diseases in the U.S. Dr. Edith Bracho-Sanchez , a pediatrician and a mom, demonstrates how to prevent tooth decay. 1. Start brushing twice a day with a fluoride toothpaste as soon as your little one gets their first tooth. A small smear is all you need until children turn 3. 2. Start flossing when your child's teeth touch. Once a day is great! 3. Allow them the room to practice and learn, but always brush and floss afterwards