Wordpress: If parent is page id, set child page theme

page.php

<?php
if (134 == $post->post_parent) {
    include (TEMPLATEPATH . '/page-mysubpagetemplate.php'); // Name this for your child page template name
    exit();
} else {
    // Do something else
    // You might want to stick your regular page.php code in here, or alternatively, you could call another template
}; ?>