BEGIN
  DECLARE FCursor SENSITIVE CURSOR FOR FStatement;
  DECLARE TEMPGUID VARCHAR(40) DEFAULT OLDROW.ID;

  PREPARE FStatement FROM 'SELECT * FROM DetailTable WHERE ParentID = ?';
  OPEN FCursor USING TEMPGUID;

  START TRANSACTION ON TABLES DetailTable;
  BEGIN
    WHILE NOT EOF(FCursor) DO
      DELETE FROM FCursor;
    END WHILE;
  
    COMMIT;
  
    EXCEPTION
      ROLLBACK;
  END;
END

Workaround:

BEGIN
 DECLARE FCursor SENSITIVE CURSOR FOR FStatement;
 DECLARE TEMPGUID VARCHAR(40) DEFAULT OLDROW.ID;

 PREPARE FStatement FROM 'SELECT * FROM DetailTable WHERE ParentID = ?';
 OPEN FCursor USING TEMPGUID;
 FETCH FIRST FROM FCursor;

 START TRANSACTION ON TABLES DetailTable;
 BEGIN
   WHILE NOT EOF(FCursor) DO
     DELETE FROM FCursor;
     FETCH FIRST FROM FCursor;
   END WHILE;

   COMMIT;

   EXCEPTION
     ROLLBACK;
 END;
END