i agree that the topic of children is a particularly thorny one for people who are trying not to fit in to The System ( ;) ). it shows in a ton of ways, and only gets more complicated as kids age and become (one hopes) more and more their own person.
one argument not to have kids (cough) is that parents don't raise kids, society raises kids (k, i'm simplifying to make a point). that is, parents are expected to, and might think that they do, have far more influence over their children than they actually have. so the idea that anarchist parents will raise anarchist, or anarchistically-inclined, children, is a misunderstanding of the situation.
i understand that i'm raising topics that are are not what you're talking about, whatever, but they're connected.
your question could be re-stated as, at what point do parent/responsible adults stop taking responsibility for the children in their life, and of course that would depend on the context/individuals/situation.
another significant metric in the topic of children is the question of raising children to be alternative and challenge the current paradigms (to the extent, again, that we even can), and how much they are raised to be functioning people. functioning in a fucked up society frequently means not being alternative (or at least not enough to threaten anything/anybody).
to me these seem like unresolvable conundrums, and having children forces one in to choosing one fucked up side or another.
anyway, these are some of my thoughts. not entirely addressing the question, certainly not as it was asked.
(note: some of my best friends are parents... )