How to display a list of posts in same child category as current post

Question

I am new to both php and wordpress. I have made some successful modifications in a child theme of Decode, but this is my first shot at a plugin. I'm trying to display a list of posts drawn from the same child category as the current post. I can produce a list of posts in the same parent category, but I can't figure out how to restrict the results of my query to the lowest level (child) category. The code below displays the type of list that I want but doesn't make the category restriction I'm looking for, and always displays an exact duplicate of the list directly under it. Any help would be greatly appreciated!

<?php

function list_subcategory_posts() {

    $post_id = get_the_ID();
    $categories = get_the_category();

    foreach ($categories as $category) {
        if (0 != $category->parent)
            $child_cat = $category;
        $current_post_type = get_post_type($post_id);

        $args = array(
            'category__in' => $child_cat,
            'post_type' => $current_post_type,
            'posts_per_page' => '5',
            'post__not_in' => array( $post_id )
        );

        $query = new WP_Query($args);

        if ( $query->have_posts() ) { ?>

            <aside class="related-posts">
                <h3>
                    <?php _e('Related Items', 'decode'); ?>
                </h3>
                <ul class="related-posts">
                    <?php while ( $query->have_posts()) { $query->the_post(); ?>
                        <li>
                            <a href="<?php the_permalink(); ?>">
                                <?php the_title(); ?>
                            </a>
                        </li>
                    <?php } ?>
                </ul>
            </aside>
        <?php }
        wp_reset_postdata();
    }
}

add_action('related_posts', 'list_subcategory_posts');
?>

Answer 1

I just figured this one out by modifying the first foreach statement.

$post_id = get_the_ID();
$cat_ids = array();
$categories = get_the_category();

 if ( $categories && !is_wp_error( $categories ) ) {

foreach( $categories as $category ) {
        if( 0 == $category->parent )
            {
             continue;
       }else{
              array_push( $cat_ids, $category->term_id ); 
            }
            $current_post_type = get_post_type( $post_id );

         $args = array(
            'category__in' => $cat_ids,
            'post_type' => $current_post_type,
            'posts_per_page' => '5',
            'post__not_in' => array( $post_id )
           );