Sarah Payne's murderer Roy Whiting has had his 50-year jail term reduced by 10 years by a High Court judge.
Whiting, 51, was not present to hear the decision on the number of years he must spend behind bars before he can apply for parole.
Sarah's mother, Sara, was in court and said she was "disappointed" by the decision and "life should mean life".
Whiting was jailed for life in 2001 for the kidnap and murder of eight-year-old Sarah, from West Sussex.
The 50-year tariff was set by the then-Home Secretary David Blunkett.
Whiting appealed against his tariff and judge Mr Justice Simon concluded the "appropriate" minimum term was a period of 40 years.
Speaking outside the court in central London, Mrs Payne said the family was "reeling" from the decision but admitted it "could have been a lot worse".
The child protection campaigner said the 50-year tariff had sent out a "good message".
"He will remain a danger to children as long as he lives and breathes," she added.
"The family don't get a parole date. There's no end to this. This is our life from now on."
After announcing the new minimum term, the judge invited the courtroom to "pause and remember Sarah Payne, who would now be 18 if she had not been murdered".
The original tariff meant Whiting would not be considered for release until he was in his 90s.
Mr Justice Simon said the time spent on remand - 234 days - must be deducted from the new 40-year minimum term.
But he stressed the sentence on Whiting remained imprisonment for life and he would be detained "unless and until the parole board is satisfied that he no longer presents a risk to the public".
"Even if the parole board decides then, or at some time in the future, to authorise his release, he will be on licence for the rest of his life," he added.
Life sentences are mandatory for people aged over 21 who are convicted of murder.
Between 1983 and 2002, home secretaries could declare the minimum term life prisoners should serve before being considered for parole.
And it was this power which David Blunkett used to impose a 50-year tariff on Whiting, virtually ensuring he would never be released.
Whiting's lawyers argued that his tariff was politically motivated and that Mr Blunkett knew his powers were about to be stripped when he imposed the 50-year term.
Judges now set the tariffs and while there are minimum term guidelines, they do not have to follow them. However, they must explain their decision if they choose not to.
Mr Justice Simon said that if Whiting's minimum term had been set under today's regime, "the appropriate starting point would have been a whole life order".
Sarah was snatched by Whiting, a convicted sex offender, in July 2000 near the home of her grandparents in West Sussex.
After a huge search, her body was discovered in a field 16 days later off the A29 at Pulborough.
Since the murder, her mother has campaigned tirelessly for victims' rights and for a change in the law to publicise where sex offenders live.