[Plugin: PHP Code Widget] Listing subpages not working

The following code, when pasted into the template sidebar, works perfectly, but doesn’t work when using this plugin.

<?php
//show parent page
$parent_title = get_the_title($post->post_parent);?>
<a href="<?php echo get_permalink($post->post_parent) ?>" class="parent"><?php echo $parent_title;?></a>

<?php
//show child pages
  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 id="subnav">
  <?php echo $children; ?>
  </ul>
  <br clear="all" />
<?php } ?>