Add child pages automatically to nav menu

I have built a custom menu in WordPress that contains links to posts and pages. I am adding it to my header using this line of code:

<?php 

wp_nav_menu(
    array(
        'theme_location' => 'primary', 
        'depth'          => 0, 
        'menu_class'     => 'nav-menu',
    ) 
);

?>

My problem is, if I add a child page to the top level ones on the menu, they don’t automatically appear as sublinks on the nav. I know I can create them manually each time by rebuilding the menu, but I would like to be able to just add a child page in the pages section and have it show up in the nav without having to go to the menu and build it there also, if that makes sense?

I’ve tried using depth => 0, but that didn’t work. Is there a way to have child pages show up without having to build it into the custom menu?

here is how:

/**
* auto_child_page_menu
* 
* class to add top level page menu items all child pages on the fly
* @author Ohad Raz <admin@bainternet.info>
*/
class auto_child_page_menu
{
    /**
     * class constructor
     * @author Ohad Raz <admin@bainternet.info>
     * @param   array $args 
     * @return  void
     */
    function __construct($args = array()){
        add_filter('wp_nav_menu_objects',array($this,'on_the_fly'));
    }
    /**
     * the magic function that adds the child pages
     * @author Ohad Raz <admin@bainternet.info>
     * @param  array $items 
     * @return array 
     */
    function on_the_fly($items) {
        global $post;
        $tmp = array();
        foreach ($items as $key => $i) {
            $tmp[] = $i;
            //if not page move on
            if ($i->object != 'page'){
                continue;
            }
            $page = get_post($i->object_id);
            //if not parent page move on
            if (!isset($page->post_parent) || $page->post_parent != 0) {
                continue;
            }
            $children = get_pages( array('child_of' => $i->object_id) );
            foreach ((array)$children as $c) {
                //set parent menu
                $c->menu_item_parent      = $i->ID;
                $c->object_id             = $c->ID;
                $c->object="page";
                $c->type="post_type";
                $c->type_label="Page";
                $c->url                   = get_permalink( $c->ID);
                $c->title                 = $c->post_title;
                $c->target="";
                $c->attr_title="";
                $c->description           = '';
                $c->classes               = array('','menu-item','menu-item-type-post_type','menu-item-object-page');
                $c->xfn                   = '';
                $c->current               = ($post->ID == $c->ID)? true: false;
                $c->current_item_ancestor = ($post->ID == $c->post_parent)? true: false; //probbably not right
                $c->current_item_parent   = ($post->ID == $c->post_parent)? true: false;
                $tmp[] = $c;
            }
        }
        return $tmp;
    }
}
new auto_child_page_menu();