Vladislava Podchapko's youngest, Daniil, a boy aged 23 months, died from hunger after six days.
Her daughter, Anna, two months short of her third birthday, remained alone in the flat with her brother's body for another three days.
The 20-year-old mother left them only candles when she locked them in her flat in central Kiev, the Ukrainian capital, before vanishing for nine days.
The girl was rushed to hospital on the point of death from starvation.
Miraculously, she is expected to survive.
"The girl was exhausted, she was very weak," said a doctor involved in her treatment.
"She was immediately taken into hospital where we started intravenous feeding.
"She is better now and we can say that she will live. We have started her on tiny amounts of solid food."
The mother has been detained and faces up to eight years in prison for child neglect.
Confronted by police, she replied: "I didn't know that children could die."
The frightened and confused children had tried to escape from the locked flat, pulling and tearing the linoleum by the front door.
Neighbours heard the children crying in the morning and evening, and there were unconfirmed claims they had called the police, who failed to respond.
In any event, the stranded children received no help, and are believed to have been found when their mother finally returned.
Police were then called.
Vladislava's social media account is full of pictures of her children from before the incident, and messages of love to them.
Ukrainian children rights envoy, Nikolay Kuleba, said the mother had taken her children out of a kindergarten.
But social services were reported to have failed to follow-up on any problems.
She had an iPhone 6 so did not seem short of money.
The children are from her previous marriage, and she since found a new boyfriend, and became pregnant by him.
Anna's future will be decided by social care workers.
"Perhaps she will be sent to an orphanage, or given for adoption, or if her father can look after her, she may go to him," said a source.
Social media comments on her page demand that Vladislava should face the death penalty for neglecting her children.
They express "disgust" at her actions in leaving her son and daughter alone.