The Newsboy Legion, along with the Guardian, were created in the early '40s and got cancelled later in the decade. Simon and Kirby also created Boy Commandos, which followed much the same arrangement of team members. A male adult, and some underage boys. I have always assumed the boys were meant to bring interest to teenage and younger boys who would identify with some kids their own age kicking the butts of criminals or America’s enemies. That feature also came to an end in the late '40s.
Jack Kirby could draw action, and perhaps it was about what he said a few times in interviews: when he was a kid he fought a lot. When people got socked in a Jack Kirby panel you know it had to hurt. In that way Jack could reminisce at the drawing board about the fists that had flown in skirmishes from his youth.
From Star-Spangled Comics #14 (1942):