Thanks! I had eventually pieced together the same explanation. So yes, I
think your fix is right, though I would write it like this:

        ret = False
        for child in self._children:
            ret |= child.cancel()
        return ret  # True if at least one child.cancel() call returned True

It would also be nice if there was a test for this behavior.

I keep worrying a bit -- a similar bug could exist in other pieces of the
code, or in other libraries. But I guess we can't do much about that.