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

http://codex.wordpress.org/Function_Reference/wp_list_pages

his code will show the child pages, and only the child pages, when on a parent or on one of the children.
This code will not work if placed after a widget block in the sidebar.

List WordPress subpages even if on a subpage

list, wordpress

PHP