How to put secondary navigation on all child pages

https://nwrdev.mambomedia.com/about-us/

On this page I’m showing the child pages in the sidebar by using the following in the sidebar.php:

<?php if(is_page('Portfolio') || is_page('Home') || is_page('Contact Us'))
  wp_nav_menu( array('menu' => 'Portfolio', 'depth' => 0 ));
  else
  echo '<ul>';
  wp_list_pages('title_li=&child_of='.$post->ID);
  echo '</ul>';?>

If you go to one of the child pages, though, the navigation is empty (no children = no pages). How do I get it to show the list of the parent’s page’s child pages (child page even though on a child page)?
I tried what was in the docs but it didn’t work.