Wordpress - List all category child post when in parent category
<p>I want to be able to list ALL posts (children and parent) when browsing a parent category.</p>

<p>Like this:</p>

<ul>
<li>Parent (Show posts from Child 1 and Child 2)

<ul>
<li>Child 1 (Show only posts from Child 1)</li>
<li>Child 2 (Show only posts from Child 2)</li>
</ul></li>
</ul>

<p>With this code below (placed in category.php) I don't get all the posts when I'm in a parent category. I just get posts from ONE child category instead of several.</p>

<p>Any ideas how to solve this?</p>

<pre><code>&lt;?php get_header(); ?&gt;
    &lt;div class="container"&gt;
        &lt;div class="row"&gt;
            &lt;?php get_template_part( 'include-cat-tag' ); ?&gt;
            &lt;div class="col-xs-12 col-sm-9 col-md-9 list-page-middle"&gt;
                &lt;header class="clearfix"&gt;
                    &lt;h1&gt;&lt;?php single_cat_title( '', true ); ?&gt;&lt;/h1&gt;
                &lt;/header&gt;
                &lt;?php
                    wp_reset_query();
                    $categories = get_the_category();
                    $category_id = $categories[0]-&gt;cat_ID;
                    $args = array(
                        'posts_per_page' =&gt; 100,
                        'category__in' =&gt; array($category_id),
                        'orderby' =&gt; 'meta_value title',
                        'order' =&gt; 'ASC',
                        'post_status' =&gt; 'publish',
                        'meta_key' =&gt; 'betyg',
                        'child_of' =&gt; $category_id
                    );
                    query_posts( $args );
                    if (have_posts()): while (have_posts()) : the_post();
                    get_template_part( 'include-list-post' );
                ?&gt;
                &lt;?php endwhile; ?&gt;
                &lt;?php else: ?&gt;
                    &lt;?php get_template_part( 'include-no-post' ); ?&gt;
                &lt;?php endif; ?&gt;
            &lt;/div&gt;       
        &lt;/div&gt;
        &lt;?php 
            get_template_part( 'include-list' );
            get_template_part( 'include-social' );
        ?&gt;
        &lt;/div&gt;
    &lt;/div&gt;
    &lt;?php get_footer(); ?&gt;
</code></pre>