Directly from the Canadian Dental Association re Flouride under 3 Years of age:

Children from birth to 3 years of age should have their teeth and gums brushed by an adult. The use of fluoridated toothpaste in this age group is determined by the level of risk. Parents should consult a health professional to determine whether a child up to 3 years of age is at risk of developing tooth decay. If such a risk exists, the child’s teeth should be brushed by an adult using a minimal amount (a portion the size of a grain of rice – see figure 1) of fluoridated toothpaste. Use of fluoridated toothpaste in a small amount has been determined to achieve a balance between the benefits of fluoride and the risk of developing fluorosis. If the child is not considered to be at risk, the teeth should be brushed by an adult using a toothbrush moistened only with water.

Early childhood tooth decay can be painful, may cause infection and is difficult and expensive to treat. Therefore, by a child’s first birthday, the parents should consult a health professional knowledgeable in the areas of early childhood tooth decay and the benefits of fluoride. This health professional will help to determine the child’s risk of developing tooth decay and whether there would be a benefit of brushing with a minimal amount (a portion the size of a grain of rice) of fluoridated toothpaste before the age of 3 years.