WordPress: Page Template: Redirect to First Child
<?php
/**
* Template Name: Redirect to First Child
*/
$id = $post->ID;
$children = get_pages([
'parent' => $id, // required
'child_of' => $id, // required
'sort_column' => 'menu_order',
'number' => 1,
]);
header('Location: ' . get_permalink($children[0]->ID));