Parentpage (Templatename: parentpage)
- Child A (Templatename: childpage)
- Child B (Templatename: childpage)
-- SecondChild B (Templatename: secondchildpage)
- Child C (Templatename: childpage)

<?php
    getchilds = array(
        'parent'        => $post->ID,
        'child_of'      => $post->ID,
        'sort_column'   => 'menu_order',
        'sort_order'    => 'ASC'
    );

    $postliset = get_pages($getchilds);
        foreach ($postliste as $post) {
        echo $post->post_content;
    }
?>

<div class="childpagestest">
[...]
        <?php the_content(); ?>
[...]
</div>