Two men burst into Edge Lane Post Office in Droylsden, Manchester, and held the weapon to the head of the boy who was standing behind the counter. They then demanded he hand over money from the till.
They also approached the Post Office counter and called for cash to be handed over.
The cashier on duty in the post office signalled the alarm, however, forcing the raiders to flee with a small amount of cash from the shop till.
Manchester Police are investigating the incident. Detective Sergeant Jed Stubbs of Ashton CID said: "It is difficult to comprehend how anyone could hold a gun to the head of an innocent child and we are determined to catch the man who did this."
Recent weeks have seen a shop assistant threatened by a knife-wielding man during an attempted robbery at a Bury St Edmunds Spar store, and an attack by masked robbers who used a crowbar to threaten staff at the One-Stop store in Alfreton, Derbyshire, before fleeing with cash.
Shop assistant Tony Frampton was hit with a hammer in an attack at Stars 'n' Jars in Glemsford, Suffolk. The raiders, who also carried a garden sickle, were fought off by 65-year-old store owner Tony Mitchell.
A recent Association of Convenience Stores survey revealed that robberies in c-stores have increased by 30% in the 12 months to March 2010.