late on Monday to postpone the execution, but to no avail.
Hussain was originally due to face the gallows in January but won four stays of execution as his lawyers fought to prove he was under 18 at the time of the offence and could therefore not be executed under Pakistani law.
An official probe into his age ruled he was an adult at the time of his conviction -- though the results have not been published officially.
British anti-death penalty campaign group Reprieve said the hanging represented "all that is wrong with Pakistan´s race to the gallows", while Amnesty International accused the government of "callous indifference" to human life.
Pakistan has hanged around 180 convicts since ending a six-year moratorium on executions in December after Taliban militants massacred more than 150 people at a school, most of them children.
Hangings were paused during the holy month of Ramadan but resumed last week. The European Union last week voiced serious concerns about the "alarming pace" of executions, warning that a prized trade status granted to Pakistan could be threatened if it breached international conventions on fair trials, child rights and preventing torture.
- Child kidnapped, slain -
Hussain, the youngest of seven children from a remote village in Kashmir, was working as a watchman in Karachi in 2004 when a seven-year-old boy named Umair went missing from the neighbourhood.
A few days later Umair's family received calls from Hussain's mobile demanding a ransom of half a million rupees ($8,500 at the time), according to legal papers.
Hussain was arrested and admitted kidnapping and killing him, but later withdrew his confession, saying he had made it under duress.
His true age has proved difficult to ascertain -- exact birth records are not always kept in Pakistan, particularly for people from poor families like Hussain's.
A birth certificate circulated in the media several months ago, but it appeared to have been issued only in December and the government said there was no proof of its authenticity.
Amnesty International estimates that Pakistan has more than 8,000 prisoners on death row, most of whom have exhausted the appeals process.
Supporters argue that the death penalty is the only effective way to deal with the scourge of militancy in the country. But critics say the legal system is unjust, with rampant police torture, poor representation for victims, and unfair trials.