The bomb was planted in the car parked on a market street in Chaman town, close to the Afghan border, and was detonated by a remote control, police said.
In the powerful blast, a child identified as Zabihullah was killed while nine others were injured, senior police official Habib Baluch said.
The blast also destroyed two vehicles and a motorcycle.
It was not yet clear who was behind the attack.
Chaman town is one of the two main crossing points for supplies for American and NATO troops fighting Taliban in Afghanistan.