The shooting occurred after the two attackers got into an argument with 18-year-old Eric Ramirez, a known gang member, then opened fire with a shotgun on a group of people as the baby was being bottle-fed inside a parked car, police Detective James Nuttall said.
Anna Contreras, 28, the pregnant woman feeding the boy, was hospitalized in stable condition, police said. The 4-month-old baby, identified as Andrew Garcia, died at a hospital.
Ramirez was wounded outside the car.
Contreras and the boy's mother have no criminal history, Nuttall said.
The attack came early Sunday after a family party was held to celebrate a baptism at a Van Nuys banquet hall.
Deputy police Chief Michel Moore said more than 40 officers were redeployed to the area to diffuse possible retaliation.
City Councilman Tony Cardenas said he would introduce a motion Tuesday to offer a $50,000 reward in the case.
Nuttall wouldn't provide further details on the investigation.