How do I apply a specific class to a PAGE and all of its CHILD PAGES

For example:

<li<?php if (is_page('12')) {echo ' class="current_page_item"'; }?>><a href="index.php?page_id=12" id="history"></a></li>

Simple enough. If this is page 12 the class current_page_item is applied.

I understand how to apply a class to a category but how would I modify the code to apply the class to a page and all of its child pages?

Rock on

J Fu