Trigger + mutation error + Cascade Update

Question

For an assignment I am trying to do a cascade update with a trigger of a primary key to its foreign keys using Oracle. I cannot use the cascade update in the foreign key.

Below is my code that generates the corresponding error:

CREATE or REPLACE trigger TR_PRODNO_FK_UPDATE
   /* trigger executes BEFORE
      an UPDATE of prodNo on the Product table */
   before update of prodNo on tbProduct
   /* trigger executes for each ROW */
   for each row

   /* begins a PL/SQL Block */
   begin
      /* update prodNo on tbProduct */
      UPDATE tbProduct a
         SET a.prodNo = :new.prodNo
         WHERE :old.prodNo = a.prodNo;
      /* update the prodNo on tbComponent */
      UPDATE tbComponent b
         SET b.prodNo = :new.prodNo
         WHERE :old.prodNo = b.prodNo;
      /* send message to output */
      dbms_output.put_line (
         '*** Related rows in the
            PRODUCT and COMPONENT table updated - TR_PRODNO_FK_UPDATE trigger');
   end TR_PRODNO_FK_UPDATE;
/

UPDATE tbProduct
    SET prodNo = '104'
    WHERE prodNo = '100';

What is going wrong here? Any ideas?