A man is due to appear in court charged with the murder of a one-year-old boy and the attempted murder of a girl believed to be the child's twin.
Both toddlers were discovered with critical injuries at a flat in Wilberforce Road, near Finsbury Park, north London, on Saturday night.
They were taken to hospital, where the boy died in the early hours of Sunday morning.
The girl, who has been operated on, remains in a critical condition, police said.
Bidhya Sagar Das, 33, of Wilberforce Road, is due to appear in custody at Thames Magistrates' Court.
A post-mortem examination in relation to the boy, who has not been formally identified, will take place on Tuesday at a central London hospital.
Witnesses heard a woman shouting for help on the street outside at around 11.10pm on Saturday and later saw two children being carried out of the property.
Mihai Manea, 29, who lives on the second floor of the white three-storey building, said the children were twins and lived on the top floor with their Romanian mother and Indian father.
A woman living opposite the building, who gave her name as Gui Gui, said she opened the window to offer help after hearing a woman shouting: "My kids."
Das was arrested in the Hackney area on Sunday evening following a major search by police.