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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)