WordPress Parent/Child Nav

gistfile1.txt

/**
 * Generate Parent/Child Page Menus
 * @link https://codex.wordpress.org/Function_Reference/wp_list_pages
 */

function print_menu_child_pages(){

  $children = '<nav class="nav-children nav-section"><ul id="menu-section">';

  global $post;

  if($post->post_parent){
    // Post Has Parent!

    // Sets Variables for determining what type of page the nav is on
    $parentID = $post->post_parent;
    $parentPost = get_post($parentID);
    $parentTemplate = get_page_template_slug( $parentID );
    $grandparentID = $parentPost->post_parent;
    $grandparentPost = get_post($grandparentID);

    if($grandparentID){
      // If Has Grandparent
        $children .= wp_list_pages("title_li=&include=".$grandparentID."&echo=0");
        $children .= wp_list_pages("title_li=&child_of=".$grandparentID."&echo=0");
    } else {
        $children .= wp_list_pages("title_li=&include=".$post->post_parent."&echo=0");
        $children .= wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
    }
  } else {

    // Post Has No Parent!
    $children .= wp_list_pages("title_li=&include=".$post->ID."&echo=0");
    $children .= wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");

  }

  // Print Subnav
  if($children){
    echo $children;
  }

  echo '</ul></nav>';

}