Hi Dominik,
It seems the way you have mapped your parent node to your child is incorrect.
Apart from mapping your need to define a method as a interface method to get/set data.
Please refer to following link for step-by-step info on how to do external node mapping. You may have missed a step.
Instantiating the Used Component.in ABAP Web Dynpro
Regards,
Karan Ranawat