Display all posts under child sub category in sidebar on post page?

I am trying to make a custom sidebar for my posts in certain categories. It will look something like this.

My category structure looks like this:

• Parent 1
  • Child 1
    • Sub Child 1
    • Sub Child 2
  • Child 1
    • Sub Child 1
    • Sub Child 2
• Parent 2

What I want to do is to display all posts under Sub Child 1 and 2. I will be more specific, lets say we have 3 posts, 2 of them is under sub child 1 and last one is under sub child 2. Then I only want the 2 posts that is under sub child 1 to be displayed under Sub child 1, and the 3 :rd post to display only under sub child 2.

I have been looking in 2 days now over the Internet to figure this out how to do this, but I dont get what I am doing wrong.

Here is my code and what I am trying to modify:

<?php 
$cat = get_the_category();
$catID = get_cat_ID($cat);
$subcats = get_categories('child_of=" . $catID);
    foreach($subcats as $subcat) {
    echo "<h3>';
    echo '<a href="' . get_category_link($subcat->cat_ID) . '">';
    echo "https://wordpress.stackexchange.com/" . $subcat->cat_name . '';
    echo '</a></h3>';
    echo '<ul>';
    $subcat_posts = get_posts('cat=" . $subcat->cat_ID);
    foreach($subcat_posts as $subcat_post) {
        $postID = $subcat_post->ID;
    echo "<li>';
    echo '<a href="' . get_permalink($postID) . '">';
    echo get_the_title($postID);
    echo '</a></li>';
    }
    echo '</ul>';
} 
?>

Any ideas?

I guess you could take a big step using wp_list_categories() with a walker to add a additional Unordered List to every item.

The code:

$cat_id = get_query_var( 'cat' );
$subcats = get_categories( 'child_of=" . $cat_id ); // child categories

class Cat_Walker extends Walker_Category {
    function end_el( &$output, $page, $depth = 0, $args = array() ) {
        $posts = get_posts( "cat=" . $page->term_id );

        if ( sizeof( $posts ) > 0 ) {
            $output .= "<ul>';

            foreach ( $posts as $post ) {
                $output .= sprintf( '<li><a href="https://wordpress.stackexchange.com/questions/89959/%1$s">%2$s</a></li>', get_permalink( $post->ID ), $post->post_title );
            }

            $output .= '</ul>';
        }

        $output .= '</li>';
    }
}

foreach ( $subcats as $subcat ) {
    $subsubcats = get_categories( 'child_of=" . $subcat->term_id ); // sub child categories

    foreach ( $subsubcats as $subsubcat ) {
        $args = array(
            "title_li'         => '',
            'show_option_none' => '',
            'taxonomy'         => 'category',
            'child_of'         => $subsubcat->term_id,
            'walker'           => new Cat_Walker( )
        );

        wp_list_categories( $args );
    }
}