grandchild pages only

<ul><?php
$postid = $post->ID;
$args = array(
'post_type' => 'page',
'post_status' => 'publish',
'orderby' => 'rand',
'posts_per_page' => -1,
'post_parent' => $postid
);
$child_level = get_posts($args);
if($child_level) :
foreach($child_level as $child_page) :

$args = array(
'post_type' => 'page',
'post_status' => 'publish',
'orderby' => 'rand',
'posts_per_page' => -1,
'post_parent' => $child_page->ID
);
$my_query = null;
$my_query = new WP_Query($args);
if( $my_query->have_posts() ) {
while ($my_query->have_posts()) : $my_query->the_post(); ?>
<li><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
<?php endwhile;
}
wp_reset_query();

endforeach;
endif;
?></ul>