The parents of a 3-year-old boy left on a bus for four hours are suing the Board of Education of Milford, Conn., and Winkle Bus Co., the school bus contractor. According to the lawsuit, the boy was left alone for four hours on June 10, 2002, when the driver became ill and drove to her home and left the vehicle in her driveway. The driver, Teresa Assunto, was arrested on a risk-of-injury charge, but court officials did not prosecute her. The incident, along with a similar one a few weeks later, prompted the school board and Winkle Bus Co. to install electronic child-check systems on all buses to remind drivers to inspect the bus after each run for any sleeping children.