rdyar, thanks for your time. If i understand you correct, with your solution i would have no content for the parents and just get the contents from the children. Anyway, i found a solution, that works.
In the parent i can know define the child-collection-name as param, like: col: col-name
Inside the parent loop i can show the child collection with this code:
{% assign col = section.col %}
{% for col in site.[col] %}
<div class="child">
<h2>Col: {{ col.title }}</h2>
</div >
{% endfor %}