Get parent pages or child pages (if exists)

Hi, I’m veeeery new to wordpress, with very basic knowledge of php.
That said ;), I can’t achieve something without using plugins that should be (I geuss) very easy to do in wp.

I would like to have a sidebar menu that:
1 When visiting main page, all top level pages are listed in the sidebar.
2 When visiting a top level page with no children, all top level pages are listed.
3 When visiting a top level page with children, just the children pages, and descendant pages, are listed.

I currently use this code, but it only does the #3.

Here is the code:

<?php /*MASSS Display child pages with thumbnail if exists*/
		if (is_page( )){$childPages = get_pages(array('child_of' => $post->ID));}
		else {$childPages = get_pages( 'include=9' );} ?>
<ul id="sidebar-childs">
    <?php foreach ($childPages as $childPage): ?>
        <li>
            <a href="<?php echo get_page_link( $childPage->ID ); ?>">
            <?php echo get_the_post_thumbnail($childPage->ID, 'thumbnail'); ?><h3><?php echo $childPage->post_title; ?></h3></a>

        </li>
    <?php endforeach; ?>
</ul>
<?php wp_reset_query(); /*MASSS END*/ ?>

(hope the `backticks work! it’s my first message, sorry)

Any help would be great!!! It’s days I’m trying to this! Thanks!!!!