Children under 3 years old may use fluoride toothpaste as soon as their teeth come in, but they should only use an amount equal to the size of a grain of rice. Older children, ages 3 to 6, should use fluoride toothpaste in an amount equal to the size of a pea. Children under 6 will need help brushing their teeth in order to avoid getting too much fluoride, which could lead to fluorosis.