Well I don't have kids, nor a younger relative who would put me in a position like this but I do have a younger sister growing up and this is what I plan on doing with her...

As long as she's mature, responsible, gets good grades, and doesn't use it for personal problems, depression or something stupid like that then I'd allow it. I'd tell her if she wants to do it, she has to smoke WITH ME, or she has to tell me when she's smoking/high. Make sure she gets her weed from me, and don't let her get it from someone who might lace it or whatever.

If she SMOKED before, then I wouldn't smoke with her unless she asks. If she SMOKES, then just blaze one day with her around and ask her if she wants to.. then explain to her if shes going to be smoking with you, then you have to lay down some ground rules *see above*.