The two boys, now 13 and who cannot be identified, were given life sentences with minimum terms of eight-and-a-half years for the murder of 19-year-old Shawn Seesahai in Wolverhampton on 13 November last year.
The solicitor general applied to increase those terms claiming they were “unduly lenient”.
Judges at the Court of Appeal have now agreed to change their sentences to 10 years.
The defendants, who were both 12 at the time of the attack, were described during their sentencing in September as the country’s “youngest knife murderers”.
Mr Seesahai was stabbed through the heart and lungs and suffered a skull fracture during the attack on Stowlawn playing fields in East Park.
One of the wounds he suffered measured 23cm deep – almost passing through his body.
They accused each other of wielding the machete but were found to be jointly responsible for the attack, which trial judge Mrs Justice Tipples called “horrific and shocking”.
The boys are believed to be the youngest convicted of murder in the UK since James Bulger’s 11-year-old killers in 1993.
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