only show children of a nav menu

Here you have the solution

 <ul>
  <?php
     $children = wp_list_pages('title_li=&child_of=".$post->ID."&echo=0');
     $subpages = ($post->post_parent) ? wp_list_pages('title_li=&child_of=".$post->post_parent."&echo=0') : wp_list_pages('title_li=&child_of=".$post->ID."&echo=0') ; 
          if ($children) { ?>
              <li><?php echo $children; ?></li>
          <?php } else { ?>
              <?php echo $subpages; ?>
  <?php } ?>
  <?php wp_reset_query() ?>
 </ul>

I used it to make a menu where the case was something like:

Parent Page: Service A
Child: Overview, Features, Service Plan, FAQs, Quotes

And there were different services and each parent has their own child.