If we're talking about weed exclusively, I would approach it the same way I would for alcohol: depends on how old the kids are, on much they are consumming, when they are consumming, and whether or not it has an effect on the rest of their life. Basically, I don't think there's any reason to freak out about a responsible 16 years old smoking a join or having a few beer now and then on the week-ends. I would however see it as a problem if the kid was to come home shit faced or completely baked, if her/his perfomance at school started to drop, or if there were any other signs that their consumption was negatively affecting their life.