Initially police said the explosion took place after some unidentified people threw hand grenades at the shop which dealt in antiques and old furniture.
But later the police said the explosion took place as a result of a gas cylinder blast inside the shop.
"The blast killed the 10-year old boy who was close by and injured 10 others including two little girls who were passing in front of the shop," a police official said.
Geo News channel reported that a deep crater had been created at the spot where the shop stood due to the explosion in the Nazimabad area.
Pakistan's biggest city and financial hub also saw a resumption of violence and target killings that have claimed 10 lives since last night.
The killings took place in a number of areas of the city.
