Thanks Jochem!
I should have asked ofcourse also if you have an example or documentation that i have not found yet. 
.
I just found this masterclass 24. Parent-child forms - Documentation / Weekend masterclass - Triggre Community and think it will help me a lot.
It would also be great to see the data model setup of the masterclass. Where can I find this?
Last question:
Can I also hide a form field based on selection of the parent field? Sometimes additional fields are needed and sometimes not.
If this is possible, how can i build this?
Thanks!