William Hague, standing in for the Conservative leader as PM’s questions was suspended, said Ivan had brought “joy and love to those around him”.
Gordon Brown said everyone’s thoughts and prayers were with the family.
Six-year-old Ivan, who had cerebral palsy and epilepsy, died at St Mary’s Hospital in Paddington, London.
Mr Brown, who suggested suspending the weekly Commons clash as a mark of respect, said every child was “precious and irreplaceable” and that the death of a child “was something that no parent should have to bear”.
The prime minister, whose daughter Jennifer Jane died aged just 10 days in 2002, paid tribute to Ivan saying: “I know that in an all too brief life, he brought joy to all those around him and I know also that for all the days of his life, he was surrounded by his family’s love.”