Navigation troubles (links disappear)

Right now, my menu looks something like this:
Link 1
Link 2
Link 3
Link 4
Link 5
Link 6

Two of the pages has child pages, and I have been trying to find a way to include them in the menu like so: When you click the parent page, the child pages becomes visible in the menu, I found a link of coding which displays the child pages underneath the menu isself (although, below the parent page was what I originally wanted).

My coding:
<?php $pg_li="page_item"; if (is_home()) {$pg_li .=" current_page_item";}?>
<ul><li class="<?php echo $pg_li; ?>"><a href="<?php bloginfo('siteurl'); ?>" title="Hem">Hem</a></li>
<?php wp_list_pages('exclude=5,6,7,12,15&sort_column=menu_order&title_li='); ?>

<?php
if($wp_query->is_page && !is_home()) {
$lp_param = "";
$lp_param = "title_li=<strong>" . __('Undersidor') . "</strong>&depth=1&child_of=" .
$wp_query->get_queried_object_id();
wp_list_pages($lp_param);
}
?></ul>

When you click a link to one of the pages that has child pages, the child pages are displayed like I want them to. But when you click a link to one of the child pages the links to the child pages disappear.

Just from looking at my coding, any ideas on what I could do?

My site: https://lavished.nu (It’s in swedish, but clicking on LAV or Annat will show you what I mean).