change_template.php

<?php

add_filter('page_template', function ($template) {
    $page = get_queried_object();
    $slugs = [];
    do {
        $slugs[] = $page->post_name;
        $page = empty($page->post_parent) ? null : get_post($page->post_parent);
    } while ($page);

    $name = sprintf('pages/%s.php', implode('/', array_reverse($slugs)));
    if ($_template = locate_template($name)) {
        $template = $_template;
    }

    return $template;
});