A teenage sex offender, who social services placed with a foster family, raped their 2 year-old son and sexually assaulted their 9 year-old daughter.
The boy, who is now 18 and admitted to the rape and indecent assault, has been locked up indefinitely for the public's protection.
The teenager cannot be named for legal reasons but it was revealed that he abused the toddler and his 9 year old sister during the several months he spent in the family's home in The Vale of Glamorgan.
During the trial, the Prosecuter described how the foster parents' daughter told them how he would come into her room, lie on top of her, kiss her and put his hands under her clothing. The abused girl told her parents that he put his hand over her mouth when she called out.
She only told her parents after her baby brother was raped.
David Pinnel, defending the teenager, said social services had to take some responsibility for the situation: 'The criminal responsibility is his but the point has to be made that it would not have happened if he had not been placed in that position.'
Social services chiefs have admitted a 'serious error of judgement' in placing the youth with a foster couple who had young children.
The young sex offender was given an indefinite sentence but told he would serve at least six years and only be released when he was 'no longer a risk'. He was also ordered to register as a sex offender and was banned from working with children for life.
During the trial it emerged that the youth had been responsible for a string of sexual approaches towards children.
 Five years previously he had faced allegations of 'sexually inappropriate behaviour' with a young boy.
In 2005 he admitted exposing himself and touching another young boy sexually while they were both living at a hostel.
He was forced to resign from a job in a bowling alley in 2007 because parents discovered he was trying to get their daughters' telephone numbers.
Last year he was caught sexually assaulting a 16-year-old girl he thought was asleep.
Senior managers have apologised to the foster parents and offered support for now and the future.
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