Display trimmed version of the_post()

First of all, never use query_posts, use WP_Query instead.
To show 50 carachters of each one of the pages:

<?php 
$args = array(
        'post_type' => 'page',
        'post_parent' => '6',
        'order'=> 'DESC'    
        );
$pages_returned = new WP_Query($args);
        while ( $pages_returned->have_posts()): $pages_returned->the_post(); ?>

<div class="page-excerpt">
     <h1><?php the_title(); ?></h1>
     <?php the_excerpt(); ?>
</div>
<?php endwhile; ?>

Now for your 50 chars limit, on your functions.php add

function custom_excerpt_length( $length ) {
    return 50;
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );

That should be it.