Adding class to menu items that have child pages

Hello, i am quite new into PHP and Themeediting but i want to enhance the Twenty twelve with some addition to menu entries that have a submenu. The theme doesnt add any class but i found a solution here in the codex:

http://codex.wordpress.org/Function_Reference/wp_nav_menu
Chapter: How to add a parent class for menu item

But it doesnt work, i copied it into an empty functions.php that i placed in my child folder, also i didnt forget the <?php ?> tag around it.

Is there anything else i need to do to make it work?

Thank you
Sofian

Here is the content of my functions.php:

<?php
add_filter( 'wp_nav_menu_objects', 'add_menu_parent_class' );
function add_menu_parent_class( $items ) {

	$parents = array();
	foreach ( $items as $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 ) ) {
			$item->classes[] = 'menu-parent-item';
		}
	}

	return $items;
}
?>