How to list child pages??

The post id is probably not available in the sidebar. Try this:

global $post;
$pages = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");

Thanks but this won’t work. It still says NULL.

<?php
global $post;
  $children = wp_list_pages('title_li=&child_of='.$post->ID.'&echo=0');
  if ($children) { ?>
  <ul>
  <?php echo $children; ?>
  </ul>
  <?php } ?>

Try this hope it may work.

Thanks but nothing changed. Still the same.

What theme are you using? Where did you download it from?

I use my own theme.

How many loops/queries are running on the page?

There are 2. Normal posts and CPT.

Are you resetting the query after each loop?

Yes, everytime. But nevermind, I use the plugin Gecka Submenu to this now. Thanks anyway!