CalculateRollupFieldRequest and Create message on Child record

var tracingService = (ITracingService)serviceProvider.GetService(typeof(ITracingService));
            var context = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));

            var serviceFactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
            var service = serviceFactory.CreateOrganizationService(context.UserId);

            var entityName = "new_childentity";

            if (context.PrimaryEntityName != entityName) return;

            var entity = context.MessageName != "Delete"
                                   ? (Entity)context.PostEntityImages["Target"]
                                   : (Entity)context.PreEntityImages["Target"];

            if (entity != null && entity.Contains("new_opportunityid")&& entity["new_opportunityid"] != null)
            {
                var opportunity = (EntityReference)entity["new_opportunityid"];

                var calculateRequest =
                    new CalculateRollupFieldRequest { Target = opportunity, FieldName = "new_rollup" };

                var calculateResult = (CalculateRollupFieldResponse)service.Execute(calculateRequest);

                var recalculatedOpportunity = calculateResult.Entity;

                var calculatedAmount =
                    recalculatedOpportunity.GetAttributeValue<Money>("new_rollup").Value;

                tracingService.Trace("New value: " + calculatedAmount);
            }