Pretty sure you cannot do this using XML Auto.
Try Using XML Path
Here's an example using Path from AdventureWorks
SELECT
h.SalesOrderID,
h.OrderDate,
h.DueDate,
h.ShipDate,
h.Status,
h.SubTotal,
h.TaxAmt,
h.Freight,
h.TotalDue,
(SELECT
CarrierTrackingNumber,
OrderQty,
ProductID,
UnitPrice,
UnitPriceDiscount,
LineTotal,
ModifiedDate
FROM [Sales].[SalesOrderDetail] d
WHERE h.SalesOrderID = d.SalesOrderID
FOR XML PATH('OrderDetail'), ROOT('OrderDetails'), Type)
FROM [Sales].[SalesOrderHeader] h
WHERE h.SalesOrderID = 43666
FOR XML PATH('SalesOrder'), ROOT('SalesOrders')
<SalesOrders>
<SalesOrder>
<SalesOrderID>43666</SalesOrderID>
<OrderDate>2001-07-01T00:00:00</OrderDate>
<DueDate>2001-07-13T00:00:00</DueDate>
<ShipDate>2001-07-08T00:00:00</ShipDate>
<Status>5</Status>
<SubTotal>6079.6842</SubTotal>
<TaxAmt>486.3747</TaxAmt>
<Freight>151.9921</Freight>
<TotalDue>6718.0510</TotalDue>
<OrderDetails>
<OrderDetail>
<CarrierTrackingNumber>D46A-40CA-8D</CarrierTrackingNumber>
<OrderQty>1</OrderQty>
<ProductID>764</ProductID>
<UnitPrice>419.4589</UnitPrice>
<UnitPriceDiscount>0.0000</UnitPriceDiscount>
<LineTotal>419.458900</LineTotal>
<ModifiedDate>2001-07-01T00:00:00</ModifiedDate>
</OrderDetail>
<OrderDetail>
<CarrierTrackingNumber>D46A-40CA-8D</CarrierTrackingNumber>
<OrderQty>1</OrderQty>
<ProductID>753</ProductID>
<UnitPrice>2146.9620</UnitPrice>
<UnitPriceDiscount>0.0000</UnitPriceDiscount>
<LineTotal>2146.962000</LineTotal>
<ModifiedDate>2001-07-01T00:00:00</ModifiedDate>
</OrderDetail>
</OrderDetails>
</SalesOrder>
</SalesOrders>