Tommy Butler, the former husband of Celia Beckett, 34, sobbed as he told Nottingham Crown Court of her "violent" nature.
Ms Beckett is accused of poisoning her daughter Tracey, four, who died in 1986 from an overdose of an anti-depressant, amitriptyline, which was prescribed to Ms Beckett and which she is alleged to have fed to the child. The cause of death was originally given as bronchitis but detectives applied to have the body exhumed last January.
Ms Beckett, of Newark, Nottinghamshire, is also charged with causing grievous bodily harm to another daughter Clare, who suffered a serious brain injury in 1984 when four months' old. Clare died in 1991 in a home for the severely mentally handicapped.
She is further charged with poisoning and cruelty to a third daughter, Debbie, six, between 1989 and 1992.
Mr Butler, 41, said he was distraught at the death of his two children. He told the court that Ms Beckett twice threatened to kill the children. "Celia said to me in confidence she couldn't stand seeing Tracey and Clare suffer and she wanted to give them some drugs to put them to sleep," he said.
Oliver Blunt QC, for the defence, rejected the claims, saying Mr Butler had not told the police about the threats but had invented them. He said Mr Butler had sold photographs of his daughters to a national newspaper and had tried to sell a book about the case.
Mr Butler admitted he had recently applied to the Criminal Injuries Compensation Board for compensation for the death of his two children.
Earlier, the court was told that Ms Beckett had blamed Mr Butler for causing the brain injury to Clare.
David Holmes, who became Ms Beckett's boyfriend in 1986, said: "Celia told me Tommy Butler had caused the injury." However, Mr Holmes described his anger at how Ms Beckett treated Tracey. He said she ignored her after the child had suffered a serious overdose, just three weeks before she died from a second, similar overdose in September 1986.
The trial continues today.Reuse content