Menu.php

<?php

if ( !defined('ABSPATH') ) exit;

class Menu {

    private $flat_menu;
    public $items;

    public function __construct( $name ) {

        $this->flat_menu = wp_get_nav_menu_items($name);
        $this->items = [];
        foreach ($this->flat_menu as $item) {
            if (!$item->menu_item_parent) {
                array_push($this->items, $item);
            }
        }
    }

    public function get_submenu( $item ) {

        $this->submenu = [];
        foreach ($this->flat_menu as $subitem) {
            if ($subitem->menu_item_parent == $item->ID) {
                array_push($this->submenu, $subitem);
            }
        }

        return $this->submenu;
    }
}