If DCSS fail to refund you in a reasonable amount of time and correct the wage assignment then you need to bring them before the court by way of motion. Consult with an attorney or the Family Law Facilitator's office at the court house.
Realistically, your chances of recovering your overpayment lies squarely against your ex/mother of your child/ren. Case law suggests that DCSS has no obligation to return any overpayment.
Consult a family law attorney for assistance with your matter, and possibly alternative means of offsetting against your overpayment.