WCF Data Services doesn't support nested aggregates. But DataServiceQuery does -- with certain gotchas.

DataServiceQuery.cs

            var result = _unitOfWork.BillingBatches
                                    .Select(batch => new BillingBatch
                                    {
                                        Id = batch.Id,
                                        Description = batch.Description,
                                        FundingSource = batch.PayerCustomer.Name,
                                        Amount = batch.Bills.Select(bill => bill.NetTotal).Sum(), // turns into $expand -- EF would have used SQL SUM()
                                        Created = batch.Created
                                    });