可用模板列表由生成get_page_templates. 此函数结束时，您将看到一个允许您修改输出的过滤器。您可以在以下特定条件下使用该选项进行更改：

add_filter (\'theme_page_templates\',\'wpse302574_conditional_templates\', 10, 4);

function wpse302574_conditional_templates ($post_templates, $this, $post, $post_type) {
  $parent_id = wp_get_post_parent_id ($post->ID);
  if (get_page_template_slug ($parent_id) == \'slug_of_your_parent_template\') {
    // remove unwanted templates from $post_templates
    }
  return $post_templates;
  }

你可以这样做

function wpdocs_filter_theme_page_templates( $page_templates, $this, $post ) {

      $parent_id = wp_get_post_parent_id( $post->ID );

      $parent_template = get_page_template_slug( $parent_id );

      if( \'template-parent.php\' === $parent_template ){ // compare parent template

        foreach ($page_templates as $key => $value) {

          if( \'template-child.php\' != $key ){ // compare child template

            unset( $page_templates[$key] ); // This will unset all the template except default template and child template

          }
        }

      }

      return $page_templates;
  }
  add_filter( \'theme_page_templates\', \'wpdocs_filter_theme_page_templates\', 20, 3 );