Function get_page_children not working

Hello,
Im getting mad with get_page_children function!
I have 2 pages with one child for each. The function works only with the second one. The code is:

<?php $all_wp_pages = new WP_Query(array('post_type' => 'page'));
	      $child = get_page_children( $post->ID, $all_wp_pages );
	?>
	<div class="link-page aligncenter textcenter" style="padding-top:20px;"><a href="<?php echo get_permalink( $child[0]->ID ); ?>">.: Itinerario dettagliato :.</a></div>

The first page return an empty array for $child.
The parent ID for the first page’s child is correct!
Why this happen?