when you have many parents the link list is too long. I prefer to have only 2 select and 2 submit button.

In module "relativity.module" I have changed few lines.
The new block is:

$new_options = "";
  $exist_options = "";
  $out = false;
  
  foreach($childrentypes as $type) {
    if (relativity_may_add_child($parent, $type)) {
      $type_name = node_get_types('name', $type);
      
      $may_create = relativity_may_attach_new_child_type($parent->type, $type);

      // only offer to let user create new child if user has 'create' access 
      // and common child relationship isn't required
      if ($may_create && node_access('update', $parent)) {
        //$output .= l(t('Create new !type', array('!type'=>$type_name)), "node/add/$type/parent/$parent_nid", array('class' => 'relativity_create_' . $type));
        $out = true;
        $new_options .= "<option value=\"".url("node/add/$type/parent/$parent_nid")."\">".t('Create new !type', array('!type'=>$type_name))."</option>";
      }

      // 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', $parent)) {
        if ($may_create) {      // If 'create' link is present ...
          //$output .= ' | ';      // insert a pipe separator between links
        }
        //$output .= l(t('Attach existing !type', array('!type'=>$type_name)), "relativity/listnodes/$type/parent/$parent_nid", array('class' => 'relativity_attach_' . $type));
        $out = true;
        $exist_options .= "<option value=\"".url("relativity/listnodes/$type/parent/$parent_nid")."\">".t('Attach existing !type', array('!type'=>$type_name))."</option>";
      }
    }
  }
  
  $new_select = "<select name=\"new_child\" id=\"new_child$fieldset\">$new_options</select>
  <input type=\"button\" name=\"submit_new\" value=\"Go\" onclick=\"location.href = document.getElementById('new_child$fieldset').value;\" />";
  $exist_select = "<select name=\"exist_child\" id=\"exist_child$fieldset\">$exist_options</select>
  <input type=\"button\" name=\"submit_exist\" value=\"Go\" onclick=\"l = location.href = document.getElementById('exist_child$fieldset').value;\"/>";
  
  if ($out) {
    $output = "\n".'<div class="relativity_add_child">'.$new_select."&nbsp;&nbsp;&nbsp;".$exist_select."</div>\n";
  }