Fixed this one by using the corresponding seblod table. In my case I used:
SELECT count(*) FROM #__cck_store_form_trajectdeel where td_parentid = $cck->getValue('tr_id');
or more universal (fill in your names):
SELECT count(*) FROM #__cck_store_form_[your-content-name] where [your-parent-ID-name] = $cck->getValue([your-row-fieldname]);
The above select will retrieve the number of child-rows per parentID