Webhook Template not picking up child records

[MikeZ]

Hi,
I have a Workflow Action to a webhook to POST to a Rest API. The json data is a parent/child. The UX interface and the underlying data entered into Google Sheets is fine. However, the webhook template is not passing any child records. Any troubleshooting hints? I deleted and recreated the workflow. I authenticate fine, and I use webhooks with the same REST API in another appsheet app, though that template doesn’t rely on the START construct.
Here’s the template that’s failing to catch child records

The child data is “Transfer Lines”

{
   "purpose": "Material Transfer",
   "name": "<<[ID]>>",
   "company": "#######",
   "posting_date": "<<[Date]>>",
   "owner": "<<[User]>>",
   "from_warehouse": "<<[From Warehouse]>>",
   "to_warehouse": "<<[To Warehouse]>>",
   "items": [
      <<Start: [Related Transfer Lines]>>
      {
         "s_warehouse":"<<[s_warehouse]>>",
         "t_warehouse":"<<[t_warehouse]>>",
         "item_code": "<<[Item Code]>>",
         "qty": "<<[Quantity]>>",
         "transfer_qty": "<<[Quantity]>>",
         "uom": "Each",
         "stock_uom": "Each",
         "conversion_factor": 1
      },
      <<End>>
   ]
}

I must be missing something
Mike