Felicia Boots was charged with the murder of her 10-week-old son Mason and his 14-month-old sister Lily at their home in southwest London last week.
The 34-year-old jewelry designer was originally to appear at the Old Bailey via video link today from the Holloway Prison in London.
But London's Metropolitan Police Service said in an email to The Canadian Press that Boots did not appear at the hearing.
It did not say why she was absent, but Britain's the Telegraph newspaper reported she was too ill to take part.
Police say the hearing has been adjourned until Aug. 10 for what they called a "plea and case management hearing."
The Telegraph also said Boots was due to be transferred to a psychiatric hospital under the Mental Health Act.
The children were found unconscious by their father, investment banker Jeffrey Boots, when he returned home from work last Wednesday.
An inquest into the deaths of the children will open at Westminster Coroner's Court on May 17, police said.
The family had moved in to their luxury home in London just two weeks ago.
British media reports have said Felicia Boots is believed to have been suffering from post-natal depression for some time.