So far, I've been really good at keeping drugs on the down low with my parents, although I have had a few close calls (take, for example, when my mom came to my room while I was on shrooms. She was concerned because I was apparently using the bathroom a lot...simply a result of drinking lots of water, I told her [and that's what it was]. My pupils were dilated as fuck, and it took all my might to stop from laughing) and I'm pretty sure they suspect it (my older brother KNOWS for sure I smoke, because I overheard him commenting to his fiance that it smelled like "pot") but I think they'd rather bury their heads in the sand about it, pretend it's not there unless they literally catch me doing it or I admit to it.
But I know what their reactions would be, judged on how my mom handled finding empty bottles of Sam Adam's: she would be very livid about it, then guilt-trip me by saying how bad of a parent she is because of something I do, my dad would simply say "your mom's very upset about this." I know my mom probably thinks marijuana leads to heroin and meth and shit because that's how it happened for my neighbor. To top it all off, she'd probably call the cops too, since she'd think of herself as aiding and abetting criminal activity by seeing the drug but not reporting it to the police. My dad would simply tell my mom, and the above would happen.
As for how I would handle it with my hypothetical child, I dunno. I think it'd have to be a case-by-case basis for me, and it all depends on age, what they're doing, and how much of it they're doing.