First off, not married, no kids (personally)...
growing up I was pretty straight edge: didn't drink till end of high school, drank in college but didn't smoke weed till senior year. My parents were pretty out of the picture when it came to being open about sex, drugs, etc. when I was younger.
But I had a friends of all sorts.
Case 1: Highschool. The son of a very famous actor/comedian. His dad smoked, kept a "secret" stash from his son, but the son would occasionally raid it. I think the dad rolled J's, maybe had a small pipe? The son had a massive bong. The dad knew, didn't do anything about it, but also didn't join in. It was an unstated thing, like an unwritten law, that it just wasn't discussed.
Case 2: Highschool. Average Joe upper middle class kid. He smoked weed and so did his dad. When we were over at his house, maybe 5 or 6 of us, his dad would be talking to us, and then we'd start hitting a bong, and he'd ask his dad if he wanted to join. His dad did, but it always felt weird, awkward, but I think this was more the dad's personality.
Case 3: Highschool. Super rich class kid. His dad smoked weed and so did he, and the dad usually had faster/better hook-ups on it. The dad was Manic Depressive and OCD. He was always jacked up on some kind of meds. You'd be over at this kid's house and the dad would come up to you and start chilling with you. He was like a big kid. Very animated, social, could talk to you about anything. Totally open, talked these 15 year olds like they were 30 and friends of 15 years. THe dad would pull out a baggie and start packing a bowl, hit it, then offer it to the kids (he knew they also smoked). It was totally morally wrong, but he just felt "cool" and it worked.
None of those kids did very well in school, and I'm pretty sure that it had to do with all the weed they smoked. I believe there is a strong correlation between smoking pot and poor memory performance. And for that reason, if I ever have a kid of my own, if it comes up, I'm going to have to impose some kind of rules about not smoking weed. I picture a scene in my head like I catch them smoking, and I'm not mad, I can't be mad at that, kids are kids... but I ask them how long they've been doing it. Hopefully I'm smart enough that I just caught them as they were getting into it, and not that they've been doing it for years under my nose. If its during summer I tell them that once summer is over they have to stop. If it's during a school year, I tell them that I'm taking a good look at their grades, and if they drop so much as a letter, they are majorly, severely grounded!
On the other hand, if they are 28 and out of school, I couldn't care less if they are smoking.