Allows for subpages to have their own unique templates in WordPress (ie. /my-page/edit/ uses page-my-page-edit.php)

wp_get_subpage_template.php

<?php
// Allows for subpages to have their own unique templates (ie. /my-page/edit/ uses page-my-page-edit.php)
add_filter('template_include', 'get_subpage_template');
function get_subpage_template($template, $page_id = null) {
  if ($page_id === null) {
    global $wp_query;
    $page = $wp_query->queried_object;
  } else {
    $page = get_post($page_id);
  }

  if ($page && $page->post_type === 'page') {
    $ancestors = get_post_ancestors($page->ID);

    if (!empty($ancestors)) {
      $new_template = get_template_directory() . '/page';

      foreach($ancestors as $ancestor) {
        $new_template .= '-' . get_post_field('post_name', $ancestor);
      }

      $new_template .= '-' . $page->post_name . '.php';

      if (file_exists($new_template)) {
        return $new_template;
      }

      return get_subpage_template($template, end($ancestors));
    }
  }

  return $template;
}
?>