Ben Hope, 39, and Jason Richards, 38, face life sentences when they are sentenced next week, the Daily Mirror reported.
Prosecutors said a businessman, whose name wasn't released, had paid the two men 1,000 pounds ($1,600) each to kill another man who owed him money, but the pair, high on drugs, carried out the bungled hit with "staggering incompetence."
Aamir Siddiqi was killed in 2010 when he opened the front door of the family home. His parents said the killers yelled loudly as they stabbed him over and over.
The teenager, who was studying for school tests, went to the door because he believed his Koran teacher had arrived, The Guardian reported.
Aamir's father and mother were wounded as they tried to protect their son.
"I think that they have done something very sinister and they will be sentenced very heavily, and that is what they deserve," his father, Iqbal Ahmad, said after the verdict.
Prosecutors said the intended target of Hope and Richards, who originally met in prison, happened to live near Siddiqi's family.
Witnesses said Hope spent his money on a new pair of running shoes and a laptop computer that he pawned a few days later to get money for drugs.
"It's a chilling thought that in this city there are two men who murdered for the price of a pair of training shoes and a laptop computer," Patrick Harrington, the prosecuting lawyer, said.