The query you want to construct will probably resemble the following,
//Module.SubOrderLines[Module.Order_SubOrderLines='ID_12345']
- The 'Module' string is the name of the module these entities are in.
- The 'SubOrderLines' and 'Order_SubOrderLines' are strings you can find the the domain model
- The 'ID_12345' string is the GUID of the Order the SubOrderLines are associated with
The XPath basically reads as 'all SubOrderLines where the associated Order is ID_12345'.