Aaron Campbell was 16 when he abducted the six-year-old girl from her bed at her grandparents' home on the Isle of Bute before raping and murdering her, on July 2 last year.
He was given a life sentence with a minimum of 27 years at the High Court in Glasgow in March.
On Tuesday, three judges ruled his minimum sentence should be reduced to 24 years on account of his age at the time of the murder.
Now, Alesha's mother Georgina Lochrane has spoken out about the sentence reduction and said she believes Campbell is "playing the system".
"I'm worried for the future generations of children who will be running about when he gets out. He's a danger to children and he's always going to be," she told the Daily Record newspaper.
"He's the worst of the worst and they've caught him on his first kill. We could be sitting here with 10 children dead if they hadn't caught him - and I don't think he would have stopped until he was caught."
She added: "People keep saying, 'It's only three years', but it's three years too much. He took a six-year-old wee girl's life and 27 years wasn't even long enough."
Ms Lochrane believes Campbell is "playing the system" and vowed to be "Alesha's voice and stand up for her and fight all the way" if he makes a bid for parole in the future.
With the sentence reduction, Campbell will be 40 before he is able to apply for parole.
Tuesday's ruling from Lord Drummond Young, Lord Menzies and Lord Justice Clerk said: "We have concluded that a punishment part of 24 years would be appropriate to reflect the appellant's youth.
"We will accordingly allow the appeal to the extent of substituting that period for the sentence imposed."
They added: "As with all punishment parts, this is not an indication of the date when the appellant will be released.
"It specifies rather the period which must pass before the appellant may even apply for parole.
"As the trial judge had observed... 'whether (the appellant) will ever be released will be for others to determine but as matters stand a lot of work will have to be done to change (the appellant) before that could be considered - it may even be impossible'."
Campbell was linked into the courtroom via video, showing no emotion during the hearing.
During the nine-day trial in March, Campbell tried to convince the jury he had sex with Toni McLachlan, the girlfriend of Mr MacPhail, on the night of the murder.
He later confessed the crime to those assessing him ahead of the sentencing, saying he was "quite satisfied by the murder".
Alesha, from Airdrie, North Lanarkshire, had been staying with her family on Bute during the summer holidays