The whole country's thoughts and prayers are with the Cameron family after the death of six-year-old Ivan, Prime Minister Gordon Brown has said.
He was speaking in the Commons as prime minister's questions was cancelled.
Mr Brown said the death of a child was "was an unbearable sorrow that no parent should have to endure".
Shadow foreign secretary William Hague thanked the PM for his words and said David and Samantha Cameron had passed on thanks for the messages of sympathy.
Ivan Cameron, who had cerebral palsy and epilepsy, died in hospital early on Wednesday. He was Conservative leader Mr Cameron's oldest child.