docs: def get_child_objective_ids(self, objective_id=None):
          """Gets the child Ids of the given objective.
  
          arg:    objective_id (osid.id.Id): the Id to query
          return: (osid.id.IdList) - the children of the objective
          raise:  NotFound - objective_id is not found
          raise:  NullArgument - objective_id is null
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method must be implemented.
  
          """
          if objective_id is None:
              raise NullArgument()
          url_path = construct_url('childids',
                                   bank_id=self._catalog_idstr,
                                   obj_id=objective_id)
          id_list = list()
          for identifier in self._get_request(url_path)['ids']:
              id_list.append(Id(idstr=identifier))
          return id_objects.IdList(id_list)