The girl was in Gallery Lane at Bayside Centre in Frankston about 12.15am on Thursday when she was allegedly approached by a 14-year-old boy, a 12-year-old girl and an 11-year-old girl.
Police said the group demanded the girl hand over her phone and other possessions before stabbing her several times with a sharp object.
Transit police officers responded to the incident and arrested the children.
The Port Melbourne girl was taken to hospital with non-life-threatening injuries.
Police have since charged a 14-year-old Frankston boy and a 12-year-old Highett girl with armed robbery and assault.
The pair have been bailed to appear at a children's court at a later date.
An 11-year-old Carlton girl was released pending summons.