The incident happened on Dallimore Circle, near Eglinton Avenue East and Don Mills Road, at around 4 p.m. ET.
A uniformed officer was the first responder on scene and performed CPR on the child.
Police said the child was rushed to Toronto East General Hospital.
It's not clear whether the parents of the child were also attending the pool party.
Police said the homicide squad has been notified but the death is not considered suspicious.Suggest a correction