All were transported to trauma centres via an emergency rush.
Earlier, paramedics said the individual children had life-threatening, serious and minor injuries, but it wasn't immediately clear which injuries were suffered by which child.
An 11-year-old boy was also struck and taken to hospital in serious condition.
It is not known if the fifth victim who was taken to hospital sustained gunshot injuries, CTV News reported.
"I really, really hope in this case, so we can bring these people to justice and apply the full weight of the law, that people who were around there or there who saw something. they will confide in the police or in Crime Stoppers so we can track these people down and deal with them", Tory said.
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"There are three children who were shot here tonight", she said.
"It seems like that they were just there and happened to be... ended up being shot by the shooters".
Taverner said they believe the suspects fled the scene using a vehicle that was seen speeding away.
Police are urging members of the public to come forward with any information they have about the incident.
In a later update, Duty Inspector Kelly Skinner confirmed that there is more than one suspect involved.
Toronto Mayor John Tory said it was "a shocking and outrageous act of gun violence" that comes amid an overall decline in shootings in the city over the past year. "There is nothing more brazen than children outside at a gathering and violence happening in front of them and them being victims", she added.
"This is a tragedy", Skinner said.
"What we're doing right now is appealing for the community". "We absolutely need your help, so please call 23 Division at 416-808-2300 if you have any information that you can provide to help us catch these suspects".