code: function(control) {
        return this._events.some(function (evt) {
           if (evt.eventType === "ChildWorkflowExecutionCompleted") {
              var initiatedEventId = evt.childWorkflowExecutionCompletedEventAttributes.initiatedEventId;
              var initiatedEvent = this.eventById(initiatedEventId);
  
              if (initiatedEvent.startChildWorkflowExecutionInitiatedEventAttributes.control === control) {
                 return true;
              }
           }
        }, this);
     }
docs: Return true if the child workflow is completed
  @param {String} control
  @returns {Boolean}