Geeshaun Wilson, 21, Jennell Moore, 23, and the boy were arrested around 3 a.m. Tuesday roughly an hour after they allegedly held up a 26-year-old man who was working inside a business in the 400 block of South Clark Street, police said.
The worker handed over money from the register and the trio fled the scene, police said.
They were taken into custody in the 1100 block of South State Street. A gun, ammunition and narcotics were recovered during the arrest, police said.
All three were charged with armed robbery.
The 12-year-old boy was also charged with aggravated unlawful use of a weapon by a person under the age of 21.
Wilson was charged with possession of ammunition without a valid FOID and possession of under 30 grams of cannabis.