Display content of specifc pages in loop

I’m looking to create a loop of child pages under a certain parent page (or any other way I can specify a group) be displayed on my index page. I have the below but I can’t figure out how to select the pages I want

<?php query_posts( 'post_type=page'); ?>
    	<?php if (have_posts()) : ?>
    		<?php while (have_posts()) : the_post(); ?>
   				<?php the_title(); ?>
    			<?php the_post_thumbnail(); ?>
    			<?php the_excerpt(); ?>
   			<?php endwhile; ?>
    	<?php endif; ?>