Scary to think Paramount who was producing the film didn't realize STAR TREK is NOT a kid's show.
Paramount knew exactly who its audience were.
While TOS spent its first three years in late prime time adult viewing time slot, but you have to remember that it created
astounding ratings and longevity for its time in the 70s and early 80s, running "stripped" (ie, five nights per week) and syndicated in an early prime time slot. In the 60s, TOS was popular with university students, who supposedly gathered for dorm parties to watch a b/w TV set together. They had no disposable income (except for those, ah, necessities of university) By 1979, though, TOS reruns were being watched by the kids of those university graduated.
So ST:TMP was expected to tap into
that new and building audience: young kids and their whole family unit, who were getting to know these characters who'd originally ran on TV before many of them were born. And now were in a show that was a healthy ratings lead-in to the news (or was alternate viewing to another channel's news).
"G" films in 1979 did
not have the stigma they do today. It was suitable for family viewing, not that TMP was necessarily terribly entertaining for some kids. Note that the Director's Edition DVD doesn't carry a "G". The addition of bonus footage, the commentary track and supplementary materials, plus changed standards, was enough to shunt it into a higher classification.
I don't understand the "G" rating in TMP as opposed to the "PG" in the other classic Trek films. The distinction appears to be arbitrary.
Films are classified by an independent arbiter, not the studio.
TMP's Billy Van Zandt (the Rhaandarite ensign) was also in "Jaws 2". During filming, he got to do
two final scenes: getting eaten by the shark and getting washed up onto the rocks. His was the pivotal death scene that would have kicked the film into an "R" rating. Because the film was about
teenagers, the movie needed to be "M", not "R", otherwise no teenagers could see the movie with their Friday night dates. So Bob Burnside got to live! The studio complied with classification guidelines to scrape into "M", rather than be labeled an "R".