BACKGROUND I have two objects in a master-detail relationship. One is Parent, the other is Child. I have a Process Builder Flow that starts when a Parent record is created or edited. I have criteria on the Process Builder Flow that when the field 'Review Date' is changed on the Parent, it has an immediate action of a Flow called 'Change Review'.
THE PROBLEM One of the Set Flow Variables I need to pass to Change Review Flow is the Id of the Children. However, I cannot access the ChildIds either as a Field Reference or in a Formula. Process Builder will not let me save when I use the formula [Parent__c].Child__r, saying that the field doesn't exist, even though it is available if I try to add an Action Type 'Update Records'.
THE QUESTION How do I pass the ChildId from the Process Builder Flow to the Flow?