Check to see if a page has children, and if it does, list pages out

child-page-listing.php

  if( !$post->post_parent ){ // If is a parent page
    	$parent_id = $post->ID;
    } else { // If is a child
    	if( $post->ancestors ) {
    		$parent_id = end( $post->ancestors );
    	}
    }

    if ($children) {
    	echo "<ul class='page-children'>";
    		wp_list_pages( "title_li=&child_of=".$parent_id );
    	echo "</ul>";
    }