change class on child pages

How to modify wordpress to display pages links like this.
I changed the class to class=”page_parent_item” but i want to modify the class on child <li>

I want something like this

<li class="pagenav"><h2>Pages</h2>
<ul>
<li class="page_parent_item">Page 1
<ul class="page_child_item">
<li class="page_item"><a href="" title="">Child 1.1</a></li>
<li class="page_item"><a href="" title="">Child 1.2</a></li>
<li class="page_item"><a href="" title="">Child 1.3</a></li>
<li class="page_item"><a href="" title="">Child 1.4</a></li>
</ul>
</li>
<li class="page_parent_item">Page 2
<ul class="page_child_item">
<li class="page_item"><a href="" title="">Child 2.1</a></li>
<li class="page_item"><a href="" title="">Child 2.2</a></li>
<li class="page_item"><a href="" title="">Child 2.3</a></li>
<li class="page_item"><a href="" title="">Child 2.4</a></li>
</ul>
</li>
<li class="page_parent_item">Page 3
<ul class="page_child_item">
<li class="page_item"><a href="" title="">Child 3.1</a></li>
<li class="page_item"><a href="" title="">Child 3.2</a></li>
<li class="page_item"><a href="" title="">Child 3.3</a></li>
<li class="page_item"><a href="" title="">Child 3.4</a></li>
</ul>
</li>
</ul>
</li>

I made all the changes in template-functions-post.php but i can’t modify the class on child pages.