How to display all child/sub pages from all parent pages?

Here’s a random thought. query for ALL pages, but exclude the parents. Something like this maybe?

query_posts( array( 'post__not_in' => array( 2, 5, 12, 14, 20 ) , 'post_type' => 'page', 'orderby' => 'rand' , 'posts_per_page' => '3' ) );