A little snippet in case you come across a theme where sub-navigation isn't showing in the mobile nav select.

gistfile1.txt

{% for link in linklists.main-menu.links %}
  {% assign child_list_handle = link.title | handleize %}
  {% if linklists[child_list_handle].links != blank %}
  <option value="{{ link.url }}">{{ link.title | escape }}</option>           
    {% for childlink in linklists[child_list_handle].links %}
    <option value="{{ childlink.url }}">&nbsp; - {{ childlink.title | escape }}</option>
    {% endfor %}
  {% else %}
    <option value="{{ link.url }}">{{ link.title | escape }}</option>
  {% endif %}
{% endfor %}