When I was around 13, I discovered that my mother smoked marijuana and I was devastated. She tried to hide it from me, and so there was that deception that gnawed at me, but I had also been indoctrinated into the "just say no" style of thinking, so I just thought all drugs were bad and harmful. Personally I think it's not really setting a great example for your kids if you do it; even if you could use the drug and hide it really well, why would you want to deceive your kids like that?
What will happen in the future if they discover that their children smoke weed? They will not be able to stop them since they will still be smoking and they may not have realised the negative side of it. Better they quit smoking before their kids realise about their smoking habit.
I feel your concern. These are obviously very young kids who don't deserve to be exposed to such kind of drugs. What if they inhale the marijuana while they sleep? Despite the love,care and affection they receive from their parents, they surely don't deserve this.