Keep subpages when your on a subpage

Hey, I’ve builded my navigation with subpages, and lets say I got a link called “About” then I have some subpages/childs to “About” … they’re called “History” and “Team”. If I go to “History” all my subpages disappear! I want to keep the subpages from “About” when I’m on a subpage too. Is that possible? My code looks like this:

<ul id="navigation">
<li><a href="index.php">Forside</a></li>
<?php wp_list_pages('title_li=&depth=1'); ?>
</ul>

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

I hope it’s possible and somebody will help me! ??