Officers have been told the boy suffered a “cut to his hand when the girl handed him a knife”.
Witnesses arrested the 15-year-old Eltham boy after he allegedly assaulted a patron and staff at a bakery, according to police.
The 13-year-old girl was also arrested by police after a member of the public followed her.
Ambulance Victoria said a woman had suffered non-life-threatening injuries during the assault and she was assessed by AV at the scene.
Paul told 3AW’S Mornings that his daughter witnessed the incident’s aftermath.
“She was walking on her break, and had asked someone what happened – because there was a commotion, she was told there was a stabbing, and I believe two people were stabbed,” he told 3AW.
“It was a private altercation. She was a minute away before it happened. She was working and was told to lock-up and go home.”
The two teens will be interviewed by police at a later date.