You can find the answer on the plugin’s page at wordpress.org. Select the tab ‘Other Notes’. You will find it at the example list:
global $post;
// WP page that is a child of page 77
return in_array(77, get_post_ancestors($post));
Edit: As an alternative to your suggestions you may try
global $post;
return !empty(get_post_ancestors($post));
This is an expensive call and should better be used for testing only. But I suspect that there might be something wrong with your page hierarchy. Is it possible that your top level page is not on root level?