An Illinois couple has filed an anonymous lawsuit alleging that a school bus driver sexually assaulted their 3-year-old twin daughters.

According to the Chicago Sun-Times, the parents say the abuse occurred between October 2011 and January 2012, when the Department of Children and Family Services launched an investigation into the driver.

Last month, that investigation substantiated allegations that the children -- who attended special education pre-school classes at Gower Elementary School in Willowbrook, Ill. -- were abused by the driver, an employee of Grand Prairie Transit.

The girls' mother previously told CBS Chicago that the abuse was deeply traumatizing.

"Every time I see a school bus I panic because I don’t know how many other kids this has happened to," she told the station.

"It just seems that their innocence has been taken from them at the age of 3," their father added.

The parents, who are seeking more than $500,000 in damages, allege that the driver adjusted his route in order to ensure that the twins were the last passengers dropped off, WGN reports.

The driver -- a Berwyn, Ill. resident -- has been ordered to stay away from the family through 2014, but no criminal charges have yet been filed against him.

The 11-count lawsuit was filed in Cook County Circuit Court.