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.
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 said she had been preparing herself for his sentence to be reduced but had not expected the three-year cut.
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.”
Alesha's uncle, Calum John MacPhail, told ITV News he was losing faith in the justice system to protect people.
He said: "We can't do anything to the system other than now what I'm doing in trying to get this law changed."
"It's very frustrating and hard to put into words how angry, upset and disappointed I am in the current justice system that would allow someone like him to have an appeal and then, two, to grant that appeal and then grant him three years less that what he was already given," Mr MacPhail added.
The family had to spend days in court listening to the details of the case, including graphic details of the 117 injuries Campbell inflicted on Alesha.
Mr MacPhail said: "There's nothing we can do and turn round and say 'do you know what, that's acceptable'.
"The justice system has been give give give to him and take away from us."
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."
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’.”
Ms Lochrane was joined by Alesha’s father Robert MacPhail in the gallery at the Criminal Appeal Court in Edinburgh when the appeal was first heard in August.
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 and was just days into her summer holidays.