As long as my kid kept up honor roll (A's and B's) and understood the dangers of other drugs, and ONLY got his or her weed from me, then I'd allow it. Once my child reaches the appropriate age where it wont fuck up growth (about 14 or 15, at least) then I'd ask him about drugs. I'd let him know about how safe weed is, and as long as he promises that it is the only thing he does, and never hides anything from me I'd allow it. I'd make sure he never carried it on him or if he did very rarely, never had paraphinilia, anything of the sort.

As long as my child would be RESPONSIBLE, then I'd allow it.