A judge has criticised a council after it allowed a mentally ill woman access to her children before she killed them.
The children usually lived with their father at another address
Vivian Gamor, 29, admitted beating Antoine, 10, and suffocating Kenniece, three, in Hackney, east London, in January, the Old Bailey heard.
Judge Peter Rook QC criticised Hackney Council, saying the tragedy could have been avoided and Gamor should not been given free access to the youngsters.
An inquiry is now under way to examine what lessons can be learned.
Mr Justice Rook QC said: "On the face of it, this terrible tragedy could have been avoided if Gamor had not been allowed unsupervised access and the children's father's grave concerns had been given weight."
He added: "There are implications for the social services."
The children's father, Jimi Ogunkoya, blamed social services for their deaths.
"I obeyed the law and let them go," he said in a court statement.
"I wish I had not done that. The system that I obeyed has frogmarched my children to their deaths.
"They assessed her and found nothing wrong. This is pure negligence, which will not be tolerated."
Gamor was detained under the Mental Health Act after the judge agreed her responsibility was diminished through paranoid schizophrenia.
Gamor's solicitor, Farhana Rahman-Cook, said: "Vivian Gamor has suffered from a rapid decline in mental health. Prior to any symptoms developing Vivian Gamor was a loving mother and an intelligent student, studying towards a masters degree.
"This very tragic case has placed a significant burden on local social services who have given full statements to the court."
Mother Nature Vivian
The court heard Gamor was sectioned for serious mental illness in early 2006 and the children moved in with their father.
She had claimed the children were not hers, that Jesus was her twin and she changed her name by deed poll to Mother Nature Vivian.
But Gamor was later released from care after doctors concluded she posed no risk to herself or others.
She was gradually granted supervised access and then, on three occasions, unsupervised access to her children.
It was on the third occasion of unsupervised access that Gamor killed her children during the night at her flat in Pembury Close.
Diane Ellis QC, defending, told the court: "There have undoubtedly been failures amongst the professionals caring for her."
Hackney and City Safeguarding Children's Board, a multi-agency body which oversees child protection, has ordered an inquiry into the deaths.
Board chairman Fran Pearson said: "This is a tragic case which has been devastating for this family, and has affected everyone involved with it."
She said lessons would be learned from the findings of the Serious Case Review, which will be published next month.
Hackney Council said it would not comment on the case until the inquiry's findings are published.
Marjorie Wallace, chief executive of the mental health charity Sane, said: "This harrowing case highlights the failure to assess the risk that Vivian Gamor posed to her family when their concerns were apparently ignored.
"In half of the rare headline cases Sane has analysed there has been a failure to heed the warnings of families, which was identified by the judge in this case as a key factor leading to this tragic and unnecessary loss of life."