Originally Posted by
Comic-Reader Lad
That issue (Action Comics 267) from 1960 was a pretty early appearance of the Legion -- in fact, it was only the Legion's 3rd appearance in comics -- and so the rules were not established in concrete yet.
In some early stories, they also referred to the Legion as coming from the 21st century instead of the 30th.
Eventually, everything got straightened out and when the Legion visited Supergirl again in a later story (Action 276, where she joined the Legion with Brainiac 5), the "children of" angle was not mentioned even when they referred to the earlier adventure from Action 267.
My speculation:
I think the idea was that Mort Weisinger, the editor of the Superman titles, felt that readers would be confused as to how the Legionnaires could be the same age when visiting both Superboy and Supergirl. Since Superboy was an adult in Supergirl's time, it would make "sense" to the young readers that the Legion would also be adults and have kids of their own.
I guess he later decided, as the Legion kept appearing more and more, not to have two Legions (parents & their children), and trusted his readers to get the idea that they could travel from the future to Superboy's era or Superman's era and still be the same age.