Found the problem:
After reading some other posts, I have realized that the child record addition happens only after the parent record is saved.
So when the workflow is triggered, the child record is not yet available.
SELECT(Child Table[Photo],[Reff Column]=[_THISROW].[ID])
in the Other Attachments, works but I have not yet got to that point where it happens in one go.
The solution I have so far:
I add the Parent, then
I have 2 actions on the Parent table
1. add the Child
2. change status column to trigger the workflow.
This way, an unlimited number of children records can be added and the workflow will be triggered manually