Loop Through Child Pages

I found this solution:

<?php query_posts(array('showposts' => 20, 'post_parent' => 7, 'post_type' => 'page')); while (have_posts()) { the_post(); ?>

<h2><?php the_title(); ?></h2>

<?php } ?>

However, how would I set post_parent to be the current page that’s using the template, instead of explicitly defining a page ID?