Police say that 25 people were taking part in a hayride at a private family function just north of Warkworth, about two hours east of Toronto.
While the hayride was travelling down a hill, the tractor driver lost control and struck a tree.
The driver and several others were thrown from the tractor.
Police say an eight-year-old Ajax boy was airlifted to The Hospital for Sick Children in Toronto with a serious leg injury.
The 52-year-old tractor driver, who was also from Ajax, was airlifted to a Kingston hospital with chest and arm injuries.
A 28-year-old man from Pickering was taken to a hospital in Peterborough with a serious leg injury.
A 28-year-old man from Windsor was treated for a leg injury at a hospital in Campbellford, which is close to Warkworth.