Pagers

I had the same probleme here. As you say, it couldn't have been to hard since the module already avails of a function ( nodehierarchy_get_children() ) which retrieves the children of a given node. Also, for getting the parent of a child could not be any simpler: just use $node->parent.
Anyway, this is the code I quickly put together. Take it as it is, but it should function. At the moment, I have this code in the template for my child content type (page-article.tpl.php). I dropped it in the content_bottom region. I'd be curious to know if block are "node-aware"... if they are then you could place this code in a block too.

<div id="content-bottom">
			  <?php
              $nodes_list_array = nodehierarchy_get_children($node->parent);
			  //print_r($nodes_list_array);
			  
			  while ($this_node = current($nodes_list_array)) {
    		    if ($this_node == $node->nid) {
                  //echo 'The current node\'s position within the array of children is: '.key($nodes_list_array).'<br />';
			      $curr_pos = key($nodes_list_array);
    		    }
    		    next($nodes_list_array);
			  }
			  
     		  $prev_key = $nodes_list_array[$curr_pos - 1]; // Get the child before the current
			  $next_key = $nodes_list_array[$curr_pos + 1]; // Get the child after the current
			  
			  //Output starts
			  $output = '';
			  $output .= '<div id="children_pager">';
			  $output .= '<span id="prev_child">'; //display the previous child link
			  if ($prev_key) $output .= '<a href="/'.drupal_get_path_alias('node/'.$prev_key).'" title="Previous child">Previous child</a>';
			  $output .= '</span>';
			  $output .= '<span id="back_to_parent"><a href="/'.drupal_get_path_alias('node/'.$node->parent).'" title="Back to parent">Back to parent</a></span>'; //display the back to parent link
			  $output .= '<span id="next_child">'; //display the next child link
			  if ($next_key) $output .= '<a href="/'.drupal_get_path_alias('node/'.$next_key).'" title="Next child">Next child</a>';
			  $output .= '</span>';
			  $output .= '</div>';
			  
			  print $output;
			  ?>
</div><!-- /content-bottom -->

Copy & Pasted: This issue pertains to the 6.x-1.x branch which is now legacy and is over one year old. If this issue is not applicable anymore, please consider closing it. Otherwise, please update this issue appropriately. Will be closed in two weeks if no response.