Heroin kid: I would talk to them. I would have a long, thoughtful discussion about why they want to try it (without getting outwardly angry). I would really want to know why they feel the need and what their expectations were of the drug. Then I would find someone that was a recovered junkie and invite them over to talk to them. That's my impulsive answer but you never know how you will handle such things until they are presented. Great questions though, both of them.
As for the Heroine, I would like to think that I would embrace the desire for experimentation from my child. I actually put my mother in this situation when I was young (not with heroine but other drugs and alcohol). She accepted that I would try these things regardless of her acceptance and thus let my sister and I do what we were going to do anyway at home. And I have to say in hind sight, she did the right thing. I feel like that experience led me to develop a better sense of self-control and judgement, because it really was up to me.