A three-year-old boy stabbed on the weekend at a birthday party in Oshawa, Ont., died Wednesday of his injuries.

Nathan Kelly died at Toronto's Hospital for Sick Children, police said in a statement. The boy's mother, Leslie Kelly, 26, was also killed in the Nov. 29 tragedy.

Relatives and police say the woman was fatally stabbed while trying to shield her husband, Ricky, from a knife-wielding attacker who showed up at a party for 11-year-old Brandon Kelly.

Ricky Kelly and his five-year-old son Riley were also injured in the incident. They are in hospital and are expected to survive, police said.

The family has decided to donate Nathan's organs "to help other children by consenting to organ donation, something that … Nathan's mother would have wanted," Durham police said in a release on behalf of the family.

Police shot and killed the attacker, identified as 47-year-old neighbour Gino Petralia, whose teenage son was at the party and was not hurt.

Petralia's son had been taken in by the Kellys after the teen's mother died in August.

The attack started after Ricky Kelly asked Petralia to leave the birthday party.

Two trust funds have been set up to help the Kelly family "through the difficult times ahead," police said in the statement.

The family asked police to thank the many friends, neighbours and strangers for their love and support throughout this most difficult time.

The release said private funeral arrangements are being made for Leslie Kelly and her son.