Problem when getting sub terms

Hi,

I'm using the following php code on my site

$header_links = menu_navigation_links('menu-services',0);
//print_r($header_links);
print theme('links', $header_links);

This show me all the terms at level 0 in my menu.

Now I tried

$header_links = menu_navigation_links('menu-services',1);
//print_r($header_links);
print theme('links', $header_links);

This does not output anything, the print_r is also showing an empty array.
Although when I put this menu as a block on my drupal site, it shows the sub items in the menu.

Why are the sub items not shown with the above code?

I also wonder, what would be the easiest way to also include all nodes with these terms automatically into this menu?