Hey @xoa,
Yes, you are correct that it does not work the way you want it. The “{{dc:form:data key="Kinder"}} is not Kiene” condition works only when submitting the form.
I have inspected your Form layout thoroughly, and here’s what I would suggest to minimize the white space as if there are no field rows in the area.
1.) Remove the Top Margin of the “Kontaktdaten” row.
2.) Add a Bottom Margin to each of the sub-select elements instead.

With it this way, the Row element occupies only 1 pixel of height when no dropdowns are visible. As soon as you select “1” or any number in the Kinder column above, the “Row Alter Kinder” Row element expands.
Check the page now as I demoed the steps above.
Cheers.