{% for entry in children -%}
  {% if entry.permalink -%}

  {% if active_entries and forloop.index == active_entries[3] -%}
    {% assign isActive = true -%}
    {% assign class = 'nav-link active' -%}
  {% else -%}
    {% assign isActive = false -%}
    {% assign class = 'nav-link' -%}
  {% endif -%}

  {% if entry.permalink contains '://' -%}
    {% assign isExternal = true -%}
  {% else -%}
    {% assign isExternal = false -%}
  {% endif -%}

  <li class="nav-item">
    <a class="{{class}}" href="{{entry.permalink}}"
       {%- if isExternal %} target="_blank" rel="noopener" {%- endif -%}>
      {{entry.title}}
    </a>
  </li>
  {% endif -%}
{% endfor -%}