WordPress template that directs page to first child.

Redirect Template

<?php 
/* Template Name: Redirect */
?>

<?php 
$args = array('child_of'=>get_the_ID(), 'sort_column'=>'menu_order', 'sort_order'=>'ASC');
$pages = get_pages($args);
$first_child = $pages[0]->ID;
$link = get_permalink($first_child);
?>

<?php 
header("HTTP/1.1 301 Moved Permanently");
header('Location: '.$link);
?>