Redirect main navigation link to subpage. To do this, Add this code to functions.php, create your main navigation page using this template, then add your selected pages as the sub-pages. When you now click on the main nav button it should redirect to your child page (or first subpage if more than one).

redirect.php

<?php
/*
Template Name: Redirect To First Child
*/
if (have_posts()) {
  while (have_posts()) {
    the_post();
    $pagekids = get_pages("child_of=".$post->ID."&sort_column=menu_order");
    $firstchild = $pagekids[0];
    wp_redirect(get_permalink($firstchild->ID));
  }
}
?>