wp_list_pages list all but top-level parent page

I am currently using this code, which is what I want, except that if I am on a grandchild page, it only displays the other grandchildren. I would like to display all the grandchildren and children of a single parent, without displaying the single parent. How can I do that?

<?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 } ?>