I looked for this all over. i was even using a redirect php script for a while but it is intended for using your main page links for linking to external links.
Here is the way to redirect to a sub page.
using the code in the link above, im just elaborating on the steps.
1) Create a new template file and paste the following code into it.
<?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));
}
}
?>
2) save and upload your new template into your theme directory.
3) Edit your Parent page and apply the “Page Template”.
Note: adding the page template is located down towards the bottom of the page. i also found it helpful to put a note in the body of the page form so that whoever edits it later wont be confused when they dont see any content.
hope this saves somebody some time!