<?php
/*
Template Name: Show Child
*/
$query = new WP_Query(array(
    'post_type' => 'page',
    'posts_per_page' => 1,
    'order' => 'ASC',
    'orderby' => 'menu_order',
    'post_parent' => get_the_ID(),
));

if ($query->have_posts()) {
    while ($query->have_posts()) {
        $query->the_post();

        header("Location: " . get_permalink());
        exit;
    }
}