Redirect parent page to its first child

Redirecting from the parent page comes in handy when you don't have actual need for the parent page, expect to group the sub pages under it.

Make a new file called page-pagename.php or page-ID.php and paste the following into it.

if (have_posts()) {
  while (have_posts()) {
    the_post();
    $descendants = get_pages("child_of=".$post->ID."&sort_column=menu_order");
    if ($descendants) {
      if (!get_post_meta($post->ID, 'dont_redirect', true)) {
        $first_child = $descendants[0];
        wp_redirect(get_permalink($first_child->ID));
      }
    }
  }
}

Source