Mr Brown said the country's prayers were with the Camerons
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.
Prime minister's question time was suspended after his death was announced and was replaced by brief tributes from each of the parties instead.
As they also paid tribute to soldiers killed in Iraq and Afghanistan, Mr Hague added: "However death comes, it is a devastating loss to the families involved."
Mr Brown, whose baby daughter died after just 10 days in 2002, told MPs: "Every child is precious and irreplaceable and the death of a child is an unbearable sorrow that no parent should ever have to endure."
Sending condolences to the Cameron family he added: "Politics can sometimes divide us. But there is a common human bond that unites us in sympathy and compassion at times of trial and in support for each other at times of grief.
"Sarah and I have sent our condolences to David and Samantha and I know the whole country - our thoughts and our prayers - are with David, Samantha and their family today."
Mr Hague, deputising for Mr Cameron who he had spoken to shortly beforehand, thanked him for his "generous and heartfelt" messages adding that the PM understood "as much as anyone" the "dimensions of this loss".
He told MPs the Camerons were also "hugely grateful" to the NHS care workers who "not only did their utmost for their son this morning but have helped every day since he was born".
"Ivan's six years of life were not easy ones. His parents lived with the knowledge for a long time that he could die young but this has made their loss no less heartbreaking," Mr Hague said.
"Ivan their son suffered much in his short life but he brought joy and love to those around him.
"As David himself has said in the past, for him and Samantha he will always be their beautiful boy."
Vince Cable, Liberal Democrat deputy leader, said he wanted to extend, on behalf of the party, "our deepest sympathy to the Cameron family for the loss of their son Ivan this morning".
He said there was something "especially sad and shocking about the loss of a child" adding: "I would simply express the hope that this is a personal tragedy that transcends all party barriers and I would simply want to express the hope that the family are given the space and there is privacy to grieve and cope with this tragedy that they have experienced."