Thats a hard question to answer. I started at 14. i guess i turned out ok. My family was/is dead against it so i never had family smoke sessions.
I think if I hide it from them (my mj consumption) it would make it weird later on & they may feel lied to...
Sometimes i flirt with the idea it might be better to smoke around them (not like literally in their face..just not hide my use at home from the start basically) so they grow up thinking its no big deal/forbidden fruit kinda thing. My cousin does that with her kids and they have turned out to be great kids. she allows them to do whatever they want (drink/smoke etc) so long as they do it at home and it works. both kids graduated with near 4.0 gpa's, honors, going to college, etc.
So im leaning towards the side of treating it like its no big deal. However being a parent you'd still have to use it responsibly so the kids dont think its ok to smoke and drive or be a fiend and go smoke mj every hr. etc.
I think it depends on the parents and their moral beliefs/teachings more so than just smoking mj.
just because a parent smokes mj doesnt mean their child will be terrible when they grow up.
As far as smoking WITH your kid goes...id only be willing once they picked it up on their own and liked it for themselves...not because i asked them to hit it, etc. iwouldnt do something like that. if they try it with a friend or whatever then thats one thing..