[support] My menu tree doesnt appear like a tree

Hi there, I am currently developing my own theme. The theme was designed by some web designers, and I had to implement it in Drupal (7) this way, well that's what I found on the internet. The menu is rendered pretty good, like: |<?php $hoofdmenu = menu_navigation_links('menu-hoofdmenu'); echo theme('links', array("links" => $hoofdmenu, 'attributes' => array('id' => 'hoofdnav', 'class' => 'radius'))); ?>| But.. This only works for "top level" menu items. Some how the sub-menu-items, or children, aren't visible now. How can I display the second level menu items too? Is this possible in somewhat the same way? Thanks in advance! Niek.