Mr Cameron said he had been overwhelmed by messages of support
Conservative leader David Cameron has returned to work, two weeks after the death of his six-year-old son.
He chaired a shadow cabinet meeting and issued a statement condemning the recent killings in Northern Ireland.
He has also been preparing for Wednesday's prime minister's questions - his first since Ivan died.
Ivan, the Camerons' oldest son, had cerebral palsy and severe epilepsy. He was taken ill on the morning of 25 February and died in hospital.
Resuming his political duties, Mr Cameron said the murders of two soldiers and a police officer in Northern Ireland in the past few days were "shocking".
Those responsible for the killings were "common criminals who have got to be hunted down, caught and imprisoned" not part of "some political movement", he said.
But he said the violence did not mean Northern Ireland was "on the brink" of a return to former hostilities and the peace process had transformed the country.
Also on Tuesday, Mr Cameron met Conservative former chancellors - including shadow business secretary Ken Clarke, Lord Lamont and Lord Howe - to discuss the state of the economy.
After Ivan's sudden death Gordon Brown, whose daughter Jennifer Jane died aged just 10 days in 2002, suspended that day's prime minster's questions as a mark of respect.
Instead, in an emotional House of Commons, there were short statements from the PM, shadow foreign secretary William Hague and Liberal Democrat deputy leader Vince Cable.
Mr Cameron and his wife Samantha held a private funeral for Ivan last week at St Nicholas Church in Chadlington, Oxfordshire.
The couple, who have a daughter and another son, said they had been overwhelmed by the letters and flowers sent in condolence.
Party officials are still working through about 11,500 messages which were sent to Conservative Central Office, many from parents who have lost a child.
Ivan was born in April 2002 and suffered from Ohtahara syndrome, a rare form of epilepsy characterised by spasms which start in the first days of life.