Check if entry has a descendant with a certain field enabled
Wow - thanks for listening ;-)

I solved it like this:

{# loop through continents #}
{% for entry in craft.entries.section('destinations').type('continent').find() %}   
    {# only show links to continent pages if it has any travelguides #}
    {% if entry.hasDescendants() %}
        {% set continentID = entry.id %}
        {% if entry.howLongWillYouBeGoing == 'mediumStay' in craft.entries.section('destinations').descendantOf(continentID) %}
            {# if subpage is marked as mediumstay, show a link to it #}
            &lt;li&gt;&lt;a href="{{ entry.url }}" {% if craft.request.firstSegment == entry.slug %}class="is-active"{% endif %}&gt;{{ entry.title }}&lt;/a&gt;&lt;/li&gt;  
        {% else %} 
            {# if it's not a mediumStay, just show the text #}
            &lt;li&gt;{{ entry.title }}&lt;/li&gt;
        {% endif %} 
    {% else %} 
        &lt;li&gt;{{ entry.title }}&lt;/li&gt;
    {% endif %}
{% endfor %}