docs: def remove_child_objectives(self, objective_id=None):
          """Removes all children from an objective.
  
          arg:    objective_id (osid.id.Id): the Id of an objective
          raise:  NotFound - objective_id 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()
          ols = ObjectiveLookupSession(self._objective_bank_id, runtime=self._runtime)
          try:
              ols.get_objective(objective_id)
          except:
              raise  # If no objective, get_objectives will raise NotFound
          ids_arg = {'ids': []}
          url_path = construct_url('childids',
                                   bank_id=self._catalog_idstr,
                                   obj_id=objective_id)
          try:
              result = self._put_request(url_path, ids_arg)
          except Exception:
              raise
          id_list = list()
          for identifier in result['ids']:
              id_list.append(Id(idstr=identifier))
          return id_objects.IdList(id_list)