Is it possible to push grandchild records into an Excel file?

When grabbing data from grandchild records, you will need to setup a table-row replication. Since we only support row replication for Excel templates, we recommend using Stand Alone Data or SOQL relationships to grab these data:

Stand Alone data relationship:https://help.nintex.com/en-US/docgensf/DeliveryStorage/StandAloneDataRelationships.htm
SOQL relationship: https://help.nintex.com/en-US/docgensf/DeliveryStorage/SOQLQueryRelationships.htm