Conditional tag in Widget Logic for checking if top level page or if subpage

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?