Please give us more information. Context is essential in FileMaker and knowing what table occurrences and relationships are involved is key. What is the base table for the grandparent layout? What is the table occurrence for the parent portal on the grandparent layout (and what is the relationship between the base table and portal table occurrence)? What is the base table for the parent layout and what is the table occurrence and relationship for the child fields in the popover in the parent layout?
Yes, the original post sounds like a soap opera script.
Learning about the real entities (things) you're trying to manage may give additional clues.
grandparent -> SIPMAClients
Parent -> SIPMAJobsTotalsTable
Child -> SIPMAReOrder
Table Occurrence for parent portal on the Grandparent layout
SIPMAJobsTotals (links SIPMAClients = SIPMAJobsTotalsTable)
Table Occurrence for the child fields in the popover
SIPMA ReOrders Create ReOrder (links SIPMAJobsTotalsTable = SIPMAReOrder and allows for the creation of records via the relationship)
Instead of soap, I found myself humming a C&W song: "I'm my own grandpa".
But I think that your basic relationships can be deduced here:
MagicKey for parent to Child would then add another occurrence of Child linked to parent by a different pair of match fields.
To add a record to child from grandparent, requires also requires selecting or creating a parent record to which to link the new child record. How do you plan to do that? A portal to parent such that you add the child from a row in that portal?
"To add a record to child from grandparent, requires also requires selecting or creating a parent record to which to link the new child record. How do you plan to do that? A portal to parent such that you add the child from a row in that portal?"
Yes, I tried copying the button from the Parent table into the portal row for the parent table on the grandparent layout. But that does not work.
Does the button appear in every row of the portal?
Does the name that appears in "Show related records from" in portal setup exactly the same as the name that appears in "Show record from" in your parent layout's layout setup?
Screen shots of your relationships and the button's script might help.