Looking to "Attach existing Parent" AS WELL AS "Attach existing Child"

Starting on line 1714, we've got:

// only show link to attach existing node when the potential child doesn't require a parent.
      if ((!relativity_requires_parent($type) || relativity_multi_parent($type)) && node_access('update', $parent)) {
        $items[] = l(t('Attach existing !type', array('!type' => $type_name)), "relativity/listnodes/$type/parent/$parent_nid", array('class' => 'relativity_attach_'. $type));
      }

Which creates a link in the 'Edit relationships block' for attaching an existing node as a parent. This link brings up the list of possible parents for the node.

I'm looking to add another link directly after this which will allow me to attach an existing node as a child, bringing me to a page of possible children for the node.

I really don't know what I'm doing, and this is the first module I've actually tried to edit, but so far I've come up with the last line shown here:

      if ((!relativity_requires_parent($type) || relativity_multi_parent($type)) && node_access('update', $parent)) {
        $items[] = l(t('Attach existing !type', array('!type' => $type_name)), "relativity/listnodes/$type/parent/$parent_nid", array('class' => 'relativity_attach_'. $type));
      }
        $items[] = l(t('Attach existing !type', array('!type' => $type_name)), "relativity/listnodes/$type/child/$parent_nid", array('class' => 'relativity_attach_'. $type));

It does add another item to the edit relationships menu, but the page it links to does not exist..

All I did was copy the item and change "parent" to "child" in the url. I assume I am missing a large detail here.. Any advice?