You can begin brushing your child’s teeth with a little water as soon as the first tooth appears. However, if you are considering using toothpaste before the child is 2 years of age, ask your dentist or physician first. If a child is older than two years of age, use only a pea-sized amount of fluoride toothpaste. A child should be taught to spit out remaining toothpaste and rinse with water after brushing.

By age six, most children are able to brush on their own although as a parent you should still supervise.

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ADA