They were not allowed to see the man police found as it could jeopardise a future court case, McCarthy said they were told.
The girls, who are cousins, were "a bit shaken up", and McCarthy's daughter doesn't want to walk anywhere by herself anymore.
"[Today] they're going to the park and she's like 'who's going to watch us?'
"It's knocked their confidence."
McCarthy said they had lived in Stokes Valley for 16 years and never had any issues in that alleyway before. She called it "very unusual".
The girls have described the flasher as Caucasian, mid to late 30s with light coloured hair, a grey T-shirt, tan-coloured jean shorts and black shoes with a green tongue.
The girls "did the right thing" by running away and getting help.
McCarthy urged anyone else who experienced something similar to contact police.
Police confirmed they were called to the incident about 4pm on Saturday, and said no offender was identified or located.