Amusingly enough, the Super Friends spinoff comic book did not come out until 1976, long after the show's first season had finished.
So the show had been just in reruns for years before DC did a comic book based on it. I believe the story goes that then-new DC Comics publisher, Jenette Kahn, was like, "Huh, why don't we have a Super Friends comic book?" and so a comic book was created.
It was written by longtime DC Comics Assistant Editor and occasional writer, E. Nelson Bridwell. Bridwell was famous for being one of the most obsessed creators ever in terms of paying attention to continuity and stuff like that. That dude was an encyclopedia.
And so, in the letter columns for the first issue of the series, after he made sure to let us know that any goofs in the first season of Super Friends happened despite his best efforts as the script coordinator. He then explains who he believes Marvin and Wendy are...
Long story short, Wendy is the niece of Harvey Harris, the guy who taught Batman to be a detective and Marvin is the son of Diana Prince, the woman who sold her identity to Wonder Woman so that Wonder Woman could use it as a secret identity. In other words, they are people with close connections to the heroes, which explains why they are willing to train them to become crime fighters.
So there ya go, Ian!
If anyone else has a comic book question, drop me a line at firstname.lastname@example.org!