If they are new to gun ownership then I think some training even if just the sort you had is important. If they have been around guns for a long time then no, it would not be needed.
I agree with you about negligence. Which is probably true for the majority of childhood accidents.
I have a feeling that at 9 you and your friends were more mature than many 15 year olds are today. That is one difference to take into account when discussing safety and guns.
Yeah, I think we pretty much agree. I would never call this Mom an asshole, if her concerns are based in a good-faith desire to keep her kids safe.