Why are the children in the photograph saluting the US Flag in that manner?

It is the Bellamy salute.(it is not the Hitler salute)

The Bellamy salute is the salute described by Francis Bellamy to accompany the American Pledge of Allegiance, which he had authored in 1892. During the period when it was used with the Pledge of Allegiance, it was sometimes known as the "flag salute". Since the Bellamy salute bore a resemblance to the Hitler Salute, President Franklin D. Roosevelt instituted the hand-over-the-heart gesture as the salute to be rendered by civilians during the Pledge of Allegiance and the national anthem in the United States, instead of the Bellamy salute. This was done when Congress officially adopted the Flag Code on 22 June 1942.