Yates was initially sentenced to seven years
A gang member who supplied the handgun used to kill Rhys Jones has had his sentence increased to 12 years.
James Yates, 21, provided the Smith and Wesson used to kill the 11-year-old in Croxteth Park, Liverpool - and helped with its disposal.
He was jailed for seven years, but his sentence was referred to the Court of Appeal by the Solicitor General Vera Baird QC as being "unduly lenient".
Yates was described as being a "willing assistant" to murderer Sean Mercer.
Yates, of Dodman Road, was found guilty of assisting an offender and possessing a banned firearm and was sentenced at Liverpool Crown Court in January.
It was successfully argued that his seven year sentence was inadequate to reflect the seriousness of the combination of the offences, and that custodial terms should have been consecutive rather than concurrent.
At a hearing in London the Lord Chief Justice, Lord Judge, sitting with Mrs Justice Rafferty and Mr Justice Henriques, agreed the sentence should have been longer.
Rhys Jones died after he was shot in a pub car park
Rhys was shot dead in August 2007 in a Liverpool pub car park as he made his way home from football training.
His mother, Melanie, and father, Stephen, who had described his original sentence as a "disgrace", were present for the hearing.
Outside court Mr Jones said: "We did think the [original] sentence was lenient considering what he had actually done. He had supplied the gun that murdered somebody.
"We are just really happy that that deterrent is there now... there is a big sentence if you get caught."
Yates was the last of those sentenced over the killing to appeal, but Mr Jones added: "It is the end of the trials and the appeals, but just around the corner some of those men will be getting out in the not too distant future, which doesn't sit very comfortably with us."
Announcing the decision earlier, Lord Judge stressed that gun crime was a "modern pestilence" and that offenders faced severe, deterrent custodial terms.
He said Rhys's killing was the culmination of years of warfare in north Liverpool between gangs of young, armed men.
"There is no point in us mincing our words. This use of guns is a modern pestilence. It produces fatalities and injuries to innocent victims."
It also caused "apprehension and fear" to the community as a whole, he added.
"The law is clear. If you choose to be loyal to a gang member who has committed murder you must, if convicted, expect a substantial prison sentence."
In September, Yates's fellow gang member Sean Mercer withdrew an appeal against his 22-year minimum jail term.
Mercer was 16 when he shot Rhys outside the Fir Tree pub in August 2007, while aiming for rival gang members.