docs: def add_child_objective(self, objective_id=None, child_id=None):
          """Adds a child to an objective.
  
          arg:    objective_id (osid.id.Id): the Id of an objective
          arg:    child_id (osid.id.Id): the Id of the new child
          raise:  AlreadyExists - objective_id is already a parent of
                  child_id
          raise:  NotFound - objective_id or child_id not found
          raise:  NullArgument - objective_id or child_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 or child_id is None:
              raise NullArgument()
          ohs = ObjectiveHierarchySession(self._objective_bank_id,
                                          runtime=self._runtime)
          if ohs.is_child_of_objective(child_id, objective_id):
              raise AlreadyExists()
          ids_arg = {'ids': []}
          for ident in ohs.get_child_objective_ids(objective_id):
              ids_arg['ids'].append(str(ident))
          ids_arg['ids'].append(str(child_id))
  
          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)