The boy, who cannot be named for legal reasons, also admitted three charges of sexual assault by touching and one of common assault on the young girl when he appeared at Llandudno Magistrates Court.
Last week the boy had denied the charges when he appeared at Caernarfon Magistrates Court. Initially defence solicitor Nicola Jones said there were medical issues involving the boy which may have explained his behaviour. But yesterday District Judge Andrew Shaw was told there had been discussions between the boy's parents, his solicitor and his doctor which had led to a change of mind and ultimately a change of plea.
"After very lengthy discussions with his doctor, who has known him for many years, it became clear there was nothing medically that could have caused this type of behaviour.
"I have spoken at length with the boy and his parents and advised them of the consequences of a guilty plea and the decision they have taken, on the basis of my advice, is that he will plead guilty to all the charges," said Mrs Jones. The boy was accompanied at yesterday's hearing by his father and grandmother.
His dad held him close during the 10-minute hearing.
A two-day trial due for March is not now needed the judge ruled.
Adjourning the matter for the preparation of reports the judge said: "It's an extremely difficult case."