first and last css classes on a dynamic sub menu

I am using the following code:

<div id="sub_menu">
	<?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 } ?>
		</div>

To list a submenu of a parent page. I want to try and figure out how to add .first and .last css classes to the menu. Is that possible?