FummeL on "Twenty Thriteen child theme excerp"

Hi, i am trying to make a template that displays all the childpages with the feautered image and the excerpt.

This is what i got:

<?php
    $child_pages = $wpdb->get_results("SELECT *    FROM $wpdb->posts WHERE post_parent = ".$post->ID."    AND post_type = 'page' ORDER BY menu_order", 'OBJECT');    ?>
    <?php if ( $child_pages ) : foreach ( $child_pages as $pageChild ) : setup_postdata( $pageChild ); ?>
        <div class="child-thumb">
            <h2><?php echo $pageChild->post_title; ?></h2>
            <a href="<?php echo  get_permalink($pageChild->ID); ?>" rel="bookmark" title="<?php echo $pageChild->post_title; ?>"><?php echo get_the_post_thumbnail($pageChild->ID, 'excerpt-thumbnail'); ?></a>

            <?php echo '<p>'.$pageChild->post_excerpt.'</p>'; ?>
        </div>
    <?php endforeach; endif;
?>

I want the image the be hard croped with 240x140 px but i can't get that to work. I also want to display the excerpt of the page child.

Ty in advance!