Sub menu with child page

Hello,

i’m a french user of wordpress, so forgive me my poor english in advance please.
I would like to have the same system like this blog : https://www.fredcavazza.net
When we select a mother page, it child pages are showed. I would like do the same on -https://new.infocompub.fr But i don’t understand how do this…

I tryed this :

<div id="nav">
<ul id="navlist">

<li<?php if(is_home()) echo ' class="current_page_item"' ?>><a href="<?php bloginfo('url'); ?>/">Accueil</a></li>

<ul>
  <?php// wp_list_pages('sort_column=menu_order&title_li='); ?>
</ul>

<?php
$post=htmlspecialchars($_GET["page_id"]);
if (is_home())
{
}
else
{
if ( $post->post_parent != 0) {
  wp_list_pages('title_li=&sort_column=menu_order&child_of='.$post->post_parent);
} else {
  wp_list_pages('title_li=&sort_column=menu_order&child_of='.$post->ID);
}
}
?>

</div>

The problem is that the parent pages don’t present on the home or all pages, and i would like to have all parent pages, and child page in submenu when we choose one of the mother page.

How do this please?

Thanks a lot ??