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