Preferably none.
First of all, the parent must be an example a child must want to aspire to. If you are not playing like a monster and make a living out of poker, better do not teach him poker as some kind of example or road that the child shall take.
The child should not be as parents, do the same things that parents do. The child shall choose his own way. If it just so happens that he gets attracted to poker on his own, so be it.
I know, ridiculous.Would love to teach my niece the game but she has no interest. Very interesting to see the variance in the replies on here. Never thought of teaching someone poker to equivalent of teaching them to smoke....