An alternative possibility, that may have the same or a similar effect, is to auto-close the children of heavily downvoted posts when they appear on the "Recent Comments" window. Adding an extra step to reply to such a post will tend to reduce the number of replies that is gets, and will clearly signal to the reader that the post is, in fact, the child of a heavily downvoted post.

I have no idea if this possibility will be better or worse than the heritable penalties (nor, for that matter, which option would be easier to implement).