Wordpress - Is there a default template file for child pages / subpages?

There is no specifica template for child pages, but you can do this pretty easily with the get_template_part() function.

First create a file called "content-child.php".

Second create a file called "content.php".

Next, inside of page.php, place this:

if( $post->post_parent !== 0 ) {
    get_template_part('content', 'child');
} else {
    get_template_part('content');
}

Anything that you want displayed on a child page will be placed inside of content-child.php. Anything you want displayed on non-child pages will be placed in content.php.

It's actually very easy, add follow code to your functions.php

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

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

Then you can prepare templates with following patterns:

• [parent-page-slug]/page.php
• [parent-page-slug]/page-[child-page-slug].php
• [parent-page-slug]/page-[child-post-id].php