Assuming you are referring to the poster I attached, that poster was advertising war bonds during World War I targeted at civilians.
It was an incentive to make civilians buy war bonds in order to finance military operations during the war. The poster features a little girl in order appeal to people's patriotism and love for their children so that they would buy bonds and thus give money to the government. Children cannot buy these so they weren't the target audience, and soldiers got that money, they didn't have to pay for it.
