Margaret White was studying at the University of Aberdeen between 1974 and 1977 when she helped out at Nazareth House in the city.
The 63-year-old told the Scottish Child Abuse Inquiry yesterday how she had never seen children being hit but she felt the way one boy was treated for bed wetting was “wrong”.
She said: “There was a young lad who did wet the bed and he was, probably, very harshly done by by the sister. He was never physically hurt.
“From what I know now, it was very wrong - a nine-year-old boy wetting the bed and being punished verbally.
“If he was dirty, we would clear away the sheets.”
The witness said the nun would call him “stupid boy” and “dirty boy”.
She added: “They would ridicule him.
“He was upset, that’s why we wanted to protect him. He would cry, he was distressed.”
Ms White said she never reported her concerns because it was “40 years ago” and that she would have been aged around 19 at the time and the nun was much older.
She described the Catholic-run orphanage as being “fun” and set up like it was family environment.
The inquiry heard Ms White did not see why nuns would try to “cover things up” and still let volunteers into the orphanage and go on summer holidays with the children.
Two other witnesses, Marion and Neil Smillie, looked after one child who was staying at the care home in Aberdeen during the early 1980s.
Mrs Smillie, 62, told how she was employed as a language therapist at the time and was helping a child who had been separated from his mother by social services to go to Nazareth House.
The inquiry heard the child’s father had poured boiling water over him as a baby and he developed behavioural issues, which took him out of school.
One instance involved him climbing on to a roof and throwing stones at staff and pupils.
The couple took him into their home when he was aged around five years old at the weekend a couple of times.
Neither of them had undergone an interview process before being allowed to take the boy away, the inquiry heard.
The inquiry continues.