Delete Child Posts

Try it like this:

$args = array( 
    'post_parent' => $parentid,
    'post_type' => 'custom-type'
);

$posts = get_posts( $args );

if (is_array($posts) && count($posts) > 0) {

    // Delete all the Children of the Parent Page
    foreach($posts as $post){

        wp_delete_post($post->ID, true);

    }

}

// Delete the Parent Page
wp_delete_post($parentid, true);