WordPress Sidebar Example, pulling in one level of Child Pages for navigation. Mimics typical Sidebar Widget classes.

gistfile1.php

<?php 
global $post;
$which_parent = empty($post->post_parent) ? get_the_ID() : $post->post_parent;
if( is_page() and get_children('post_status=publish&post_type=page&numberposts=1&post_parent='.$which_parent) ):
?>
<li id="widget-child-nav" class="widget">
  <a href="<?php echo get_permalink($which_parent); ?>">
    <h2 class="widgettitle"><?php echo get_the_title($which_parent); ?> Menu</h2>
  </a>
  <ul>
    <?php wp_list_pages('title_li=&depth=1&child_of='.$which_parent); ?>
  </ul>
</li>
<?php endif; ?>