def index(self, item):
        """Finds the child index of a given item, searchs in added order."""
        index_at = None
        for (i, child) in enumerate(self._children):
            if child.item == item:
                index_at = i
                break
        if index_at is None:
            raise ValueError("%s is not contained in any child" % (item))
        return index_at
Finds the child index of a given item, searchs in added order.