WordPress Loop Code to WordPress Widget

Ok.. here goes.

Lets say I have the following code working to show the child pages of a parent page (and inline child pages of current child page):

<?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 } ?>

How can I turn this into a wordpress widget/plugin so that all I have to do to make it visible on a page is drag and drop the new, corresponding widget into the sidebar?

Thanks,

Phil