The girl, 12, was charged with assault with a weapon Jan. 5 in Newfoundland, the Canadian Broadcasting Corp. reported.
She has since pleaded guilty and has been placed on 12 months probation and is now at home with her mother.
However, while she was in custody, the girl was held in an adult prison, her mother said.
"Instead of giving the youngster the help she needs and getting her assessed, they throw her in an adult prison for two weeks," the girl's mother said.
Justice officials said youths can be held in jail up to four days after their first court appearance, but must be kept separate from adult prisoners. Carol Chafe, Newfoundland's youth advocate, said prison experiences can be stressful for young people.
"Sometimes when people are in lockups, they could be under the influence of alcohol," Chafe said. "They could be belligerent; that's not nice things for a young person to listen to."
The mother said she feels the treatment of her daughter was excessive.