Page Template: Redirect Parent Page to First Child Page

Set a page to having this page template and it will show the first child page.

<?php
/*
Template Name: Go to first child
*/
	$pagekids = get_pages("child_of=".$post->ID."&sort_column=menu_order");
	if ($pagekids) {
		$firstchild = $pagekids[0];
		wp_redirect(get_permalink($firstchild->ID));
	} else {
		// Do whatever templating you want as a fall-back.
	}
?>