Wordpress - Add Descendants As Submenu Items: Remove Grandchildren

/**
 * Loop through all nav menu items checking whether the functionality has been enabled or not for them.
 * If enabled, add in submenu items for all of their descendants (only one level deep).
 *
 * @since 1.0.0
 *
 * @param array $items Array of nav menu items.
 *
 * @return array Potentially modified array of nav menu items.
 */
public function add_children_to_menu( $items ) {
    $menu_order   = count( $items ) + 1000;
    $filter_added = false;

    foreach ( $items as $item ) {
        if ( ! $this->is_menu_item_supported( $item->db_id, $item->type, $item->object ) || ! $this->is_enabled_for_menu_item( $item->db_id ) ) {
            continue;
        }

        // Get only immediate descendants (children)
        switch ( $item->type ) {

            case 'post_type':
                // Using get_pages() with 'depth' set to 1 to get only immediate children
                $children = get_pages( array(
                    'child_of'    => $item->object_id,
                    'post_type'   => $item->object,
                    'sort_column' => 'menu_order, post_title',
                    'depth'       => 1, // Limit to one level deep
                ) );

                $parent_field = 'post_parent';
                break;

            case 'taxonomy' :
                // Using get_terms() with 'parent' set to $item->object_id to get only immediate children
                $children = get_terms( $item->object, array(
                    'parent' => $item->object_id,
                ) );

                $parent_field = 'parent';
                break;
        }

        if ( empty( $children ) || is_wp_error( $children ) ) {
            continue;
        }

        // Menu items are being added, so later fix the "current" values for highlighting
        if ( ! $filter_added ) {
            add_filter( 'wp_nav_menu_objects', array( &$this, 'fix_menu_current_item' ) );
        }

        // Add each child to the menu
        foreach ( $children as $child ) {
            $child        = wp_setup_nav_menu_item( $child );
            $child->db_id = $child->ID;

            if ( empty( $child->status ) ) {
                $child->status = 'publish';
            }

            $this->added[ $child->ID ] = true; // We'll need this later

            // Set the parent menu item.
            $child->menu_item_parent = $item->ID; // Children

            // The menu_order has to be unique, so make up new ones
            // The items are already sorted due to the get_pages() or get_terms()
            $menu_order ++;
            $child->menu_order = $menu_order;

            $items[] = $child;
        }
    }

    return $items;
}