how to highlight child nav link?

I’m having a side bar navigation menu -replicate of the main navigation menu
How can I get the child nav link highlighted when I’m on a child page.
Here is what I’ve go so far

<div class="menu_side">
<?php
  if($post->post_parent) {
  $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
  $titlenamer = get_the_title($post->post_parent);
  }

  else {
  $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
  $titlenamer = get_the_title($post->ID);
  }
  if ($children) { ?>
  <ul>
<li id="menu_sideheading"><?php echo $titlenamer; ?></li>
  <li><?php echo $children; ?></li>
  </ul>
</div>
<?php } ?>

Help! Please

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