An American judge agreed to reduce his sentence on a teenage girl who bullied a toddler, provided her mother cut off her pony tail in court.

The 13-year-old girl was sent to juvenile court in Utah after admitting cutting off a three-year-old's hair.

The mother, Valerie Bruno, agreed, but has now filed a formal complaint against Judge Scott Johansen.

When the 13-year-old faced the judge he ordered she serve 30 days in detention and performed 276 hours of community service, but he also offered to take 150 hours of service off the sentence if her mother cut her ponytail in his courtroom.

Ms Bruno says she now regrets not consulting a lawyer before taking her daughter into the court.

"I guess I should have went into the courtroom knowing my rights, because I felt very intimidated," she said. "An eye for an eye, that's not how you teach kids right from wrong."

Mindy Moss, mother of the three-year-old whose hair was cut off, said she approved of the sentence and even spoke up during the hearing when she felt Ms Bruno had not cut off enough of her daughter's hair. The judge then directed her to cut the ponytail all the way "to the rubber band."

Ms Moss said she originally called police about the haircut because she worried the girls' behaviour could become more serious.

"I didn't want them to think they got away with it ... It was malicious," she said.