Sharon Dalson, 24, of Tottenham, north London, responded to voices in her head telling her to kill, the court was told. In August 1992, she attempted to hang her six-year- old son, Jason, probably with loudspeaker wire. When that failed, she strangled him.
In the same attack she smothered her five-year-old daughter, Natalia. In her diary she had written: 'It came and saw me, the evil beast and told me to do it - kill kill kill.'
Dalson, who was suffering from acute psychotic schizophrenia, was sent to Rampton high security hospital indefinitely after she admitted the manslaughter of her children on the grounds of diminished responsibility. Her plea of not guilty to their murder was accepted by the court.
The Old Bailey was told that Haringey social services failed to take action to protect the children, despite evidence that, on the days leading up to the killings, neighbours heard loud and harrowing screams from the children.
Eleven days before the children were killed, Jason was heard shouting 'No Mummy please, no', Martin Heslop, for the prosecution, said. But police who answered a 999 call found everything apparently normal. No action was taken.
Days later, long and frightening high-pitched screams were again heard in Dalson's home. The worst came just before the children died. Jason shouted and screamed for between 15 and 20 minutes.
After the children's naked bodies were found lying side by side in bed on 23 August, Dalson said: 'The voices - they made me do it. Oh God, I have killed them. They made me - it was not me.' Then she developed a mesmeric stare and would say no more.
Police found gas escaping in her home and plastic bags put against a door. She had written a letter to her mother asking for herself and her children to be buried together.
The court heard Dalson had a long history of psychiatric problems and came to the attention of Haringey social services in September 1988, after her children were found at home alone in a distressed state.
They were put on an 'at risk' register until June 1991. By the end of that year, Dalson's psychiatric condition had worsened and in February 1992 she was made the subject of a Mental Health Act hospital order after a conviction at Haringey Magistrates' Court for assault and affray.
In that incident, police officers who were called to her home found her wielding a large kitchen knife and threatening to wound herself and the children.
Jason and Natalia were then put in the care of Dalson's mother, who lived near by. Haringey social services, which did not apply for a formal care order for the children, approved the informal arrangement.
'They remained under the supervision of the social services and were again put on the 'at risk' register, but the informal care agreement was allowed to continue,' Mr Heslop said. 'It is clear that the children spent much of the time back at home living with their mother - a fact which was reported to the social services. But it seems they took no steps to ensure they were returned to the grandmother.'
Judge Neil Denison, the Common Serjeant of London, who heard the case, said two psychiatrists believed Dalson was suffering from severe mental illness and presented a clear danger to herself and to others.
A Haringey council spokeswoman yesterday refused to release the full findings of an independent review of the case commissioned by the area child protection committee. But she claimed it concluded that no one could have predicted the events that took place.
'The review has highlighted some areas where procedures could be improved. In particular, the allocation of resources for child protection in a period of change needs to be carefully considered,' she said.
Some recommendations had already been implemented, the spokeswoman said. The others would also be put into effect.
The National Association for Mental Health (MIND) yesterday called for an independent investigation, in particular into what level of professional support Sharon Dalson was receiving for her mental health problems.