If the kido is to young dont tell him anything it's none of his/her buisness and he/she will accept it (cause there kids woohoo!!). but once they start to understand them retarded anti-marijuana commercials I think it's important to step in and let them know whats up for real. I think whenever "when" they are mature enough you can explain what was once none of there bees-wax.

You dont want the kids telling your teachers and friends your parents smoke pot. It being illegal and all. never know kids are kids