Enumerate wp nav menu items. Inomplete ancestry detection; only detects parent <-> child, so 1 level.

hip_enumerate_nav_menu_items.php

<?php
add_filter( 'wp_get_nav_menu_items', 'hip_enumerate_nav_menu_items', 10, 3 );
function hip_enumerate_nav_menu_items( $items, $menu, $args ){	
	if( !is_admin() ) :			
		$i = 1;
		foreach ( $items as $item ) {			
			if( $item->menu_item_parent == 0 ) {
				$child_i = 0;						
				$item->classes[] = "parent-menu-item-$i";			
				if( $i == 1 ) $item->classes[] = "parent-menu-item-first";
				++$i;
			} else {			
				++$child_i;						
				$item->classes[] = "child-menu-item-$child_i";
			}			
		}			
	endif;	
	return $items;
}