WordPress Menu add class to submenu.

functions.php

add_filter( 'wp_nav_menu_objects', 'ns_add_menu_parent_class' );
function ns_add_menu_parent_class( $items ) {
    $parents = array();
    foreach ( $items as $item ) {
        //Check if the item is a parent item
        if ( $item->menu_item_parent && $item->menu_item_parent > 0 ) {
            $parents[] = $item->menu_item_parent;
        }
    }

    foreach ( $items as $item ) {
        if ( in_array( $item->ID, $parents ) ) {
            //Add "menu-parent-item" class to parents
            $item->classes[] = 'menu-parent-item-submenu';
        }
    }

    return $items;
}