The call came in around 8:45 a.m. Tuesday from the Llanberris Apartments on the 100 block of Conshocken State Road.
Authorities say three adults were also sickened from the exposure.
"She came out with two kids and they kind of collapsed right there on the pavement," said witness Stephanie Bell.
The Lower Merion Fire Department says the fumes came from Building C of the complex .
"We found high levels of carbon monoxide in building C. We immediately went to other buildings and found levels of carbon monoxide in those as well," said Chief Chas McGarvey.
The building was evacuated and medics transported the five victims to local hospitals. No word on their victims.
PECO and firefighters then ventilated the building. They believe the leak was caused by a stove but the incident remains under investigation.