Child Menu for Wordpress

gistfile1.php

<?php
	global $wp_query;
  	if( empty($wp_query->post->post_parent) ) {
		$parent = $wp_query->post->ID;
  	} else {
	    $parent = array_reverse(get_post_ancestors($post->ID));
	    $parent = $parent[0];
  	}
  	if(wp_list_pages("title_li=&child_of=$parent&echo=0" )) {
	 ?>
	  	<ul class="sub-nav">
	<?php
    	wp_list_pages("title_li=&child_of=$parent&echo=1" );
    ?>
    	</ul>
    <?php
  	}
?>