Three people on the street and the owner of the Toyota 4Runner were also injured. She was forced into the car's back seat during the carjacking, police said.
One of the men was armed with a gun and a baseball bat, witnesses said.
The two carjackers abandoned the Toyota and ran off. No arrests had been made Friday afternoon.
"Bottom line, we will use all the resources we have at our disposal, that's the bottom line," Police Commissioner Charles Ramsey said.
A $40,000 reward had been posted for information leading to arrests.
The carjacking took place in North Philadelphia shortly after 11 a.m.
One of the dead children was described as a 10-year-old boy. The injured, all in critical condition, were two adults, another child and the woman who had been driving the Toyota.