Magistrate Anthony Yuen Wai-ming said the case was serious, pointing out that adults who committed robberies armed with knives were liable to sentences of six to seven years.

And when young people got into such trouble, he said, they affected their families.

The incident occurred on June 1. The pair and another girl got into the taxi at Cheerful Garden in Siu Sai Wan, which drove to Kwai Chung Sports Ground. When they were at residential blocks close to the destination, the 15-year-old told the driver to stop. The two took out their knives. The 13-year-old took HK$660 and a mobile phone from the driver. The trio then left.

Police caught the two who were sentenced yesterday, while it is understood that police did not press charges against the third girl, also 13, who surrendered the next day.

The two sentenced said they had committed the robbery to help a friend pay off debts.

A lawyer for the 15-year-old said it was difficult to find positive things to say about the girl from reports about her that the magistrate had ordered, while the 13-year-old's lawyer said the younger co-defendant's reports described many negative things about her.

The 15-year-old admitted that she had held a knife and committed the robbery, the reports said.

The 13-year-old said the original plan was to steal just the cash, and admitted that it was her idea to steal a mobile phone as well.

The elder girl's mother, who runs a handmade-jewellery business, said she had started working from home so she could be with her daughter more.

Her daughter wrote in a mitigation letter that she realised that she had failed to live up to her mother's expectations and that her impulsive behaviour had caused a lot of trouble. In the past, she had not known how to tell right from wrong.

The mother of the 13-year-old told the court in a letter that it was difficult for her to accept that her daughter had done wrong. Her child had been well behaved but had made friends who were a bad influence, she said.

Ken Chan Kam-ming, chief officer for children and youth service at the Council of Social Service, expressed concern about the increasing number of teenagers aged 16 or below arrested for serious crimes. He said the figure in the first half of the year was up about 50 per cent from last year.

He attributed the trend to poor family education, and called on parents to maintain good relationships and communication with children.

He said it was wrong for teenage offenders to think they would be pardoned because of their youth.

Mr Chan called on the government to educate children on crime when the reformed education structure came into force next year.