<?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 } ?>