wp list_pages function: show children of current parent only

Hi all, i used a lot the function wp_list_pages to display an usable navigation menu.

Actually I use

<?php if ( is_page() ) : ?>
<?php $parentTitle = get_the_title($post->post_parent);?>
<?php echo $parentTitle; ?>
<?php
if($post->post_parent)
$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
else
$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
if ($children) { ?>
<ul class="subpages">
<?php echo $children; ?>

<?php } ?>
<?php endif; ?>

But it doesn’t works well with my last website because it shows all the subpages and not only the children of the current page.
I try to explain better, the pages are:

• parent 1

• child 1 of parent 1
• child 2 of parent 1
• child 3 of parent 1

• parent 2

• child 1 of parent 2
• child 2 of parent 2
• child 3 of parent 2

• parent 3

• child 1 of parent 3
• child 2 of parent 3
• child 3 of parent 3

I want to see always parents, but when I’m on child 2 of parent 3 I want to show only the children of parent 3
like:

• Parent 1
• Parent 2
• Parent 3

• child 1 of parent 3
• child 2 of parent 3
• child 3 of parent 3

Now if I’m on child 2 of parent 2 I see also all other children.

Any help?

Thanks in advance