Posts on Pages removes Sub Navigation Menu

Following the instructions given on this thread: http://wordpress.org/support/topic/show-only-x-category-posts-on-page?replies=9

I managed, with a few styling errors to get posts to display on a page… however when I do this my code which displays a navigation menu in side bar if the page has child categories disappears.

Menu Code is:

<?php
$children = wp_list_pages('title_li=&child_of='.$post->ID.'&echo=0');
if ($children) { ?>
<h2>The Little Sub Menu</h2>
    <ul>
        <?php echo $children; ?>
    </ul>
<?php } ?>

I’ve tried installing the ‘Pages Posts‘ plugin too but get the same disappearing menu problem.

Any ideas as to why it would affect this?