I won't be allowing my children to smoke marajauna in my presence, playing "Cool Mum" in my opinion is one of the worst things you can do.
I'm pretty liberal in my views towards drugs, I've tried most things as has my partner so I guess I'm resigned to the fact that our son will be curious. My approach will be one of honesty not horror stories nor being "cool" with it.
I like this exchange from South Park....
Stan: I've been told a lot of things about pot, but I've come to find out a lot of those things aren't true. So I don't know what to believe!
Randy: Well, Stan, the truth is marajauna probably isn't going to make you kill people, and it most likely isn't gonna fund terrorism, but, well son, pot makes you feel fine with being bored, and it's when you're bored that you should be learning some new skill or discovering some new science or being creative. If you smoke pot you might grow up to find out you aren't good at anything.
If that fails you can always turn to Cartman...
"Drugs are bad because if you do drugs you're a hippy, and hippys suck!"