I brush my toddler's teeth twice a day. As soon as they have teeth you should try to brush them at least once a day. Don't just let them suck on the toothbrush, either. It's the brushing motion more than the toothpaste that helps fight cavities. Also, make sure you are using toothpaste that is safe for toddlers. Toothpaste that whitens teeth and whatnot is not appropriate for children.