Don't know if it has been said, but if my kids were to start riding, I'd rather guide them than have them learn the hard way or from squidly friends. It's just like sex, I will talk to them instead of having them learn stupid nonsense from their friends. Saying it's bad and that they should refrain from doing it is not going to help.
I know for a fact that when I started riding I was clueless, and all I got from my parents was "you're going to die!". It would had been much safer if I had someone to guide me through the process, I actually learnt through internet forums. I started riding with a cheap Chinese helmet and no other gear, it wasn't until I found internet forums that I learnt about helmet certifications and proper gear.
It actually doesn't matter if you ride or not, if they get bit by the motorcycle bug they will ride wether you allow it or not. My brother doesn't ride and doesn't want anything to do with motorcycles, on the other hand, I don't even own a car! Of my riding buddies, none have parents that ride or that ever rode, we just got bit by the riding bug.