"PG" doesn't mean the same thing as it does now; PG movies weren't necessarily for kids, they just didn't have explicit sex, violence and profanity, the same as PG-13 movies today. You could show breasts (Airplane, right in a close up too) and have some pretty grusome violence (Jaws), with healthy mounts of mild profanity (most movies before the 80s...American Graffiti). In fact, some movies rated G back then had nudity (Andromeda Strain) and were intended mainly for adults (2001, Star Trek The Motion Picture).
That's why Star Wars, with its moderate cartoon violence and brief realistic violence, was rated G at first. A brief glimpse of a severed arm, some roasted corpses, exploding pilots and mildly unrealistic gunplay were things that it wasn't unusual to have in a G rated film. All a G rating was was that it just meant that if you brought kids it wouldn't be terribly objectionable. A PG film didn't mean it was for kids, it just meant it wasn't The Exorcist or Taxi Driver. Anyway, with all that Star Wars got a G rating, but in one of--or possible the--only exceptions to the ratings appeal process, Lucasfilm argued for a HIGHER rating. Their reason?
Star Wars wasn't supposed to be a kids film and they felt that by having it G rated it would be seen as juvenile by association. They wanted it to appeal to older people, so they asked the rating be raised to PG, the same rating shared by recent hit films like Jaws, Rocky and Logans Run (which had frontal nudity I think).
That's why it always seems weird the whole "Star Wars is for kids" thing. They changed the rating so it would be seen as for adults too. Parental Guidance meant the film wasn't going to be The Exorcist but as a parent you had to consider if you wanted your kids exposed to it. Heck, even after they introduced the PG-13 and parents felt like they were suddenly absolved from responsibility because they drew an arbitrary age line now, you still had PG movies like Beetlejuice where its really morbid stuff and sexual innuendo and the character yells the word FUCK. I'm sure there's another example of fuck being in a PG movie after 1984, but it just goes to show you that before the more modern era from the 90s onward, PG was seen as something that was for adults but that it could also be appropriate for some (older, usually) children to see.