“You are dealing with a case where there is no evidence at all that Mrs Clarence would have harmed a hair on their head but for the illness they suffered,” Jim Sturman QC appealed to the judge.
He said it was not right to say Clarence would have preferred “nature to take its course” when the parents just wanted their children to be happy.
Sturman said Clarence and her husband had objected to their four-year-old daughter Olivia being given a gastrostomy because they wanted her to feed normally “at meal times with the whole family sat at the table together”.
Clarence has admitted to killing Olivia, four, and three-year-old twin boys Ben and Max at the family home in New Malden, south west London.
Sturman criticised the actions of an “inexperienced social worker” who had replaced the one who had resigned “in disgust” after being moved off their case because she was an “advocate for the family”.
Shortly before the killings, and while Mr Clarence was away, she had broached the subject with Clarence that the twins, as well as Olivia, should undergo the gastric procedure next.
Opening the case for the prosecution, Zoe Johnson QC heard that all three of the children had suffered from the condition SMA type 2, and had the Clarences known before the twins were born prematurely on a family holiday to Portugal, they would have agreed to abort the pregnancy.
Outlining the events leading up to their deaths, she said Clarence’s husband had taken their eight-year-old daughter on a holiday to South Africa on Friday April 18, leaving the defendant alone with her other children, having given a nanny a day off.
Johnson said: “We suggest that the defendant took this opportunity to end the lives of her three disabled children, although it was right to say her settled intention was only formulated on the Monday.”
According to Clarence, the killings took place some time between midnight and 3am on April 22, Johnson said.
“She smothered the boys first whilst they were sleeping using a nappy so they would not smell her.
“She found it much harder to kill Olivia, and wrote a letter to her husband in the time between killing the boys and killing Olivia.”
The deaths were discovered later that night after the family nanny, called Jade, was phoned by Clarence’s mother who had tried to contact her during the day but without success.
Accompanied by her friend Daniel Magagnin and a pastor, they let themselves into the house and found Clarence in her bedroom.
Johnson said: “They got to her bedroom door and she opened the door a fraction. She was holding a towel. Mrs Clarence kept asking them to leave, and leave her alone. She was whispering.
“Daniel asked Mrs Clarence if she wanted him to pray for her. She told him to go and she was a private person. Mrs Clarence started to say ‘it’s too late, it’s too late, there’s nothing you can do to help them’.”
Jade gave Clarence a hug. Mrs Clarence lay down and pulled the duvet over her. Daniel noticed blood on her wrist and called 999.
“He opened the twins’ door and discovered the dead bodies of the twins. It was such a grotesque sight Mr Magagnin could not bring himself to look for Olivia as he was instructed to do by the emergency operator.”
Meanwhile, the court heard Clarence handed her nanny a letter, which read: “I’m so sorry I had to do this but I could not carry on. I also could not leave the children with Gary. It would have been too much for him.
“You have been such an amazing person in our lives over the last few years. Without you I most likely would have done this a lot sooner.”
By the time police arrived at the house in Thetford Road, Magagnin had become hysterical as he pointed a female police officer towards the little bodies of the twins in their bed, with toys by their heads.
Last month the Crown Prosecution Service accepted Clarence’s guilty plea to the manslaughter of the three children by reason of diminished responsibility.
Justice Nigel Sweeney is expected to hand her a hospital order on a later date.
Children with SMA type 2 have weakness in their muscles which means they cannot stand unaided, may have difficulty with other movements and are vulnerable to respiratory infections.
Although it remains a life-limiting condition, improvements in care mean that most of those affected can live long and fulfilling lives.