Help w/ Sub-Navigation Script?

I’m currently using this script for my side sub-navigation on pages.php:

<?php
if ($post->post_parent)	{
	$ancestors = get_post_ancestors($post->ID);
	$root = count($ancestors) - 1;
	$parent = $ancestors[$root];
} else {
	$parent = $post->ID;
}
$children = wp_list_pages("title_li=&child_of=". $parent ."&echo=0");
if ($children) { ?>
	<ul>
		<?php print $children; ?>
	</ul>
<?php } ?>

How can this script be tweaked so that no children pages are shown until you actually go down into one of the subcategories or sub-subcategories?

So you would initially see:

Subcategory 1
Subcategory 2
Subcategory 3

If you clicked on Subcategory 2 (or any of its sub-subcategories), you would see:

Subcategory 1
Subcategory 2
– Sub-subcategory 1
– Sub-subcategory 2
– Sub-subcategory 3
Subcategory 3

Any help would be greatly appreciated. Thank you!