Manipulate Child Pages to Use Parent Page Templates Automatically

functions.php

<?php 
// Manipulate Child Pages to Use Parent Page Templates Automatically
function switch_page_template() {
    global $post;

    // Checks if current post type is a page, rather than a post
    if (is_page()) {
        
        $current_page_template = get_post_meta($post->ID, '_wp_page_template', true);
        if (!$current_page_template) {
            $parent_page_template = get_post_meta($post->post_parent, '_wp_page_template', true);
            $parents = (is_page() && $post->post_parent == $post->ID) ? true : false;

            if ($parents == true) {
                update_post_meta($post->ID, '_wp_page_template', $parent_page_template, $current_page_template);
     
            }
        }
    }
}
add_action('save_post', 'switch_page_template');
add_action('the_post', 'switch_page_template');
?>