Mr Justice Ognall warned the man, Steven Heaney, that there must be a "profound anxiety as to the continuing risk you present" if releasing him was ever considered.
Afterwards police revealed Heaney is to be questioned in connection with at least one other crime - a sexual assault on two schoolboys at a fishing pond in Lytham St Anne's, Lancashire, in 1990.
Heaney, 37, a loner who lived with his parents in Eastham, Merseyside, had initially denied murder when Paul, 13, was found stabbed to death and 12-year-old Robert, known as Robbie, strangled and stabbed.
But he admitted the killings when his trial began at Liverpool Crown Court yesterday, more than nine months after the boys' bodies were discovered by a pond near the M53 motorway in Eastham.
Mr Justice Ognall told him: "You put a brutal end to two innocent young lives. The medical reports indicate that you are fully responsible for what you did."
Richard Henriques QC, prosecuting, said in the days running up to the murders Heaney had engaged in befriending children near his home, in at least one case using a water-testing device which was probably a sham.
He approached two 13-year-old boys and told them about nearby badgers and owls and arranged to meet them the next day. They took a 12-year-old girl with them who Heaney attempted to grab. Both the boys declined his offer to go with him in his car.
Then on the day of 29 July, he met Paul Barker and Robbie Gee and directed them to meet him at one of the ponds. He separated the boys, who had been friends since infants school, by suggesting to Paul he went to look at another pond.
"At that pond, it is beyond question that the defendant strangled Paul Barker with a wire ligature that several times encircled his neck and was pulled tight from behind," Mr Henriques said. Paul was also hit several times around the head with a mallet, suffering a fractured skull, and was stabbed nine times with considerable force.
Mr Henriques said Heaney must then have returned to the other pond where he strangled Robbie, an only child, with his hands and stabbed him 10 times in the chest.
Both boys had their trousers pulled down, their shirts pulled up and baseball caps over their eyes when they were found during a police search the next day. Heaney had gone for adrink at his local pub after carrying out the killings.
David Steer QC said Heaney, a factory worker, regretted his actions and felt contempt for himself.
The defendant had a hitherto unblemished character, and the court heard he had a record of running a "good detachment" with the army cadets for several years where the children had responded well to him.
Mr Steer added: "These were, of course, dreadful deeds for which the sentences are fixed by law. We fear that anything we have to say will pale into insignificance."
Neither the Gee nor the Barker family felt able to attend court yesterday, but will be sent transcripts of the 30-minute hearing. Detective Constable Paul Tomlinson, one of the police liaison officers who looked after them, said: "Their lives have been devastated by what happened."
A Ministry of Defence spokeswoman said they had no record of any suggestions of wrongdoing on Heaney's army cadet-force records.Reuse content