Fifteen-year-old Himel left his home on his bike on Thursday and never returned
Police said the incident occurred around 8:30am Thursday at the town's Jail Gate area.
The victims, between 6 and 8 years old, studied at a local kindergarten in the neighbourhood.
Cox’s Bazar police's Sub Inspector Md Faruq said the bus hit the children when they were crossing the road leaving 7-year-old Ashika Sultana killed on the spot.
Two others, 8-year-old Rumi Akter and her 6-year-old sister Urmi had been rushed to local hospital. They were later transferred to the Chittagong Medical College Hospital.
Police have seized the bus but the driver managed to flee, said police office Faruq.