public function applyToChildNodes($parentId, Closure $callback)
	{
        $ids = $this->findChildNodeIds($parentId);

        // If there's at least one child id, then start working next
        if (!empty($ids)) {
            foreach ($ids as $id) {
                // Invoke a callback supplying a child's id
                $callback($id);
            }
        }

        return true;
    }