Five people were treated at the scene for minor injuries, and police say three were sent to hospital.
The youngest passenger, a 3-year-old girl from Howick was taken to hospital in London with serious, but non-life threatening injuries,
Police say the buggy was travelling southbound on McIntosh Line when a piece of equipment failed on the buggy and the driver lost control.
The horse drawing the buggy was later euthanized. The road was closed for approximately three hours, but police say it has since been re-opened.