Northampton County District Attorney Stephen Baratta said the victim, believed to be about 15 months old, was pronounced dead.
She was found unconscious on Monday inside the bus in the parking lot at the Residence Bala Cynwyd on Lancaster Avenue.
With summertime comes summer heat, and an increased risk of temperature-related injuries. Outside temperatures rise dramatically, but experts say vehicle temperatures can rise 20 degrees in just 10 minutes, even on mild days, putting children at serious risk.
Police arrested a man they say is responsible for the death of his 18-month-old son after he left him "helpless in a hot truck" for three hours while he went drinking.
The family began searching for the girl about two hours after they got home and found her in the vehicle, the police said.
A 9-month-old and a 22-month-old died in separate incidents after being left in a hot car for hours in the Texas heat.