It happened Saturday afternoon at the Kiwi Yogurt shop at the Providence Towne Center near Route 29 and Route 422 in the Collegeville section.
Investigators say the shop opened around 12:00 p.m. and the robber entered the store about five minutes later.
Police say the man walked up to the counter and pulled out a gun, resembling an MP5-type machine gun, out of a computer bag.
The man ordered the customers and employees to get down on the ground.
As the employees fled to a back room, surveillance video shows a man and a small child running from the store.
Once in the back room, police say the man ordered a female employee to open the safe, but she told him she was unable to open it.
The man said "that's a shame," according to police, and demanded she give him money from the register.
The suspect then fled with approximately $305.
The suspect is described as a young white male standing about 5'10" tall. He had a slender build and was wearing a black hooded sweatshirt, black pants, sunglasses and "skater" style sneakers.
The man got in a blue sports car and drove away from the area. Witnesses described the suspect vehicle as a blue Audi TT with a loud or modified exhaust system. The man appeared to be alone.
The vehicle is believed to have been observed shortly after the robbery travelling West on High Street in Lower Providence Township.
Anyone with information is asked to contact Upper Providence Police at at (610) 933-7899.