Kirubakaran Nantheesparan, a family friend of the shop owners, witnessed the shootings. She said: "They were screaming at each other and throwing the bottles. Then I saw one pull out a gun and fire the shots.
"I saw the gun right next to me. I heard the shots fired. At first we thought they had been hit by bottles but there was too much blood. We didn't know that the girl had been shot. She was lying down in the shop in shock.
"The girl was lying on the ground and the mum ran over to her. She screamed 'call the police, call the police', there was so much blood. It was everywhere.
"She's a little girl. She was in shock and on the ground, not saying anything."
Local resident Mareh Silva, 34, was coming out of the shop with friends at about 9pm and said she saw three black youths, faces covered with scarves and balaclavas drop their bikes outside.
"I looked in and saw a lot of blood on the floor but I didn't want to look at what had happened and I was very scared," she said.
Stockwell Road was sealed off at both ends while forensic officers searched for evidence.
Detective Chief Inspector Tony Boughton said that the male victim lived in a flat above the shop while the girl was with her mother visiting the store owner, whom they are related to.
Mr Boughton said: "This is a very serious case. They (the victims) could have died and may still do. If you fire a gun into a crowded shop, there is a chance you will kill someone.
"The assumption is at the moment is that the firearm is fired through the open door because there is no damage on the outside."