How do you only show children of “active” parent using categories?
I think you don't need or want the {% nav %} tag. This should work with your current nav:


{% set categories = craft.categories.group('products') %}

&lt;ul id="sideNav"&gt;
  {% for category in categories %}
    {% set thisIsActiveCategory = (category.slug == craft.request.segment(2)) %}
    &lt;li{% if thisIsActiveCategory %} class="active"{% endif %}&gt;
      {{ category.title }}
      {% if thisIsActiveCategory %}
        {# loop over the subcategories #}
        {% set subCategories = craft.categories.descendantOf(category) %}
        &lt;ul&gt;
          {% for subCat in subCategories %}
            &lt;li&gt;{{subCat.title}}&lt;/li&gt;
          {% endfor %}
        &lt;/ul&gt;
      {% endif %}
    &lt;/li&gt;
  {% endfor %}
&lt;/ul&gt;