<?php
// Use this function to add Parent and Child Page Templates
// Structure: 
// theme_folder/page-templates/parent_page_slug/page.php - Parent Template
// theme_folder/page-templates/parent_page_slug/page-child_page_slug.php - Child Template
// theme_folder/page-templates/parent_page_slug/page-child_page_ID.php - Child Template

add_filter(
    'page_template',
    function ($template) {
        global $post;
        if ($post->post_parent) {
            
            // get top level parent page
            $parent = get_post(
                reset(array_reverse(get_post_ancestors($post->ID)))
            );

            // or ...
            // when you need closest parent post instead
            // $parent = get_post($post->post_parent);

            $child_template = locate_template(
                [
                    'page-templates/' . $parent->post_name . '/page-' . $post->post_name . '.php',
                    'page-templates/' . $parent->post_name . '/page-' . $post->ID . '.php',
                ]
            );

            if ($child_template) return $child_template;
        } elseif (file_exists(get_stylesheet_directory() . '/page-templates/' . $post->post_name . '/page.php')) {
            $parent_template = locate_template(
                [
                    'page-templates/' . $post->post_name . '/page.php',
                ]
            );

            if ($parent_template) return $parent_template;
        }
        return $template;
    }
);