Before you get the answer, you should know the history. This is a recommendation that changed back in 2014. Prior to 2014, it was not recommended to use fluoride toothpaste in children under the age of 2. Because small children don’t know how to spit out toothpaste, the fear was that they would ingest the fluoride, which would possibly lead to fluorosis. Fluorosis is a discoloration in the teeth due to too much fluoride being deposited in the tooth crystals as teeth form. They recommended brushing with water, or one of the training toothpastes that have no fluoride. Then from 2 to 6 years old, a pea sized application of fluoride toothpaste was recommended, and children should spit out excess paste as soon as they are capable.
But in 2014, clinical research prompted changes to these recommendations. Yes fluorosis was still a concern, but so was the higher rates of decay when fluoride was not used. The research found that using a small smear of fluoride toothpaste, about the size of a grain of rice, would be enough to help prevent decay, yet not so much that it would result in fluorosis in most cases, even when swallowed. Then from the ages of 3 to 6, a pea sized application of fluoride toothpaste should be used because children at that age are more capable of spitting out the extra paste.
So, the answer is, you should use a rice sized smear of fluoride toothpaste from the time the first tooth erupts until age three, and then from three to six, a pea sized application is recommended.