The 36-year-old from Pontypridd and two women had been due to stand trial at Cardiff Crown Court.
Watkins pleaded guilty to attempted rape and sexual assault of a child under 13 but not guilty to rape. This was accepted by the prosecution.
Sentencing will take place on 18 December.
Prosecuting barrister Chris Clee QC told the court: "Ian Watkins was the lead singer in a successful band called the Lostprophets.
"He accepts he was a determined and committed paedophile."
Watkins also admitted three counts of sexual assault involving children and six involving taking, making or possessing indecent images of children and one of possessing an extreme pornographic image involving a sex act on an animal.
The evidence against the defendant came from computers, laptops and mobile phones.
One laptop seized from Watkin's home was password protected with an obscenity relating to sexual activity with children, which was uncovered when it was sent to GCHQ to be cracked.
The court heard that he filmed and kept the episodes of abuse - which took place in various hotels in London and south Wales - which were recovered by police. Some of the evidence was too extreme and distressing to report.
Referring to two women standing alongside Watkins in the dock, Mr Clee said: "Both women sexually abused their own children and made them available to Watkins for him to abuse."
The court was told how the abuse of the children by all parties was also evident in text messages.
Watkins sent a text to one of the women saying: "If you belong to me, so does your baby."
In an exchange involving the other woman, she sent Watkins a message along with an image of her child to the effect that her child needed to know she was not loved.
Watkins also planned to "teach" the babies how to take drugs, the hearing was told.
Mr Clee said drugs played "a significant part in his offending against children".
Police found meth, cocaine and GHB during their searches.
Mr Clee told the court drugs were a "regular theme" in his dealings with the two other defendants.
Claiming his innocence, Watkins had told police during interviews he was being stalked by "a crazed fan" and other people had access to his computers.
He claimed he was the "victim of a malicious campaign".
The court heard in Oct 2006, Watkins met up in a hotel with a 16-year-old girl from Boston, who he had met during during a Lostprophets concert.
She dressed in a schoolgirl's outfit and he filmed their encounter.
In Oct 2008 he filmed himself having sex with another 16-year-old girl, who was also a Lostprophets fan.
One of the two women charged alongside Watkins, Woman A, admitted the attempted rape of a baby after denying rape and two charges of sexual assault as well as taking and distributing an indecent photograph of a child.
Woman B pleaded guilty to conspiring to rape a child, three sexual assault charges and four charges of taking, possessing or distributing indecent images.
After hearing legal argument from barristers, Mr Justice Royce called the jury into court and told them: "There will not be a trial in this case. The defendants have all pleaded guilty to nearly all the charges against them. The prosecution does not seek a trial in these circumstances.
"You have been saved from having to watch extremely graphic and distressing material."
Mr Clee told the judge there had been an argument regarding what some key video evidence showed.
"From the footage, there is an argument as to whether the full offence (of rape) is made out," he told judge Mr Justice Royce.
"If it is made out, it is minimal. There is so little difference between the full attempt and the attempt as to make no difference."
Watkins used his fame to abuse young children, police said. Sally O'Neill, defending Watkins, said the singer was under the care of a psychiatrist.
|Watkins, Lead Singer of The Lostprophets|
"There is no doubt in my mind that Ian Watkins exploited his celebrity status in order to abuse young children. "Today's outcome ensures the three people responsible have been brought to justice."
"Two very young children have been removed from this abuse and given a future that would otherwise have been denied them. "The investigation has been extremely complex and challenging with key information and evidence being identified from witnesses worldwide."
He added that today's guilty pleas did not mark the end of the investigation and the force would work "tirelessly" to identify other victims or witnesses.
Lostprophets guitarist Lee Gaze expressed his relief on Twitter that the case had been concluded, writing: "That was over quick.... Thanks for the kind words. At least there is closure now."
See also: http://northwoodssaveachild.blogspot.ca/2013/11/lostprophets-splits-in-wake-of-horrid.html