Updating and Adding child custom object records to an Opportunity via Web Services

I am trying to add a child custom object record to an existing Opportunity using the updateOpportunity Web Service.  I am able to update existing child objects by using the fusion 'Id' within the web service.  I had expected that if I did not include an 'Id' with the child segment, Fusion would Add the child to the opportunity rather than try to update an existing child record.  This behavior works correctly in the updateSalesParty Web Service.  When I try to do this in the updateOpportunity WS, I get an invalid key error.

As an alternative, I looked at using the Sales Custom Object Service - CreateEntity but that appears to only be available for top level custom objects and not child custom objects.