After months of heated debate, the lower House of Representatives adopted the legislation by a large majority, making the largely Catholic country the second after the Netherlands to allow mercy-killing for children, and the first to lift all age restrictions.
The ground-breaking legislation was adopted by 86 votes in favour, 44 against and with 12 abstentions. Belgium is one of three countries in Europe to allow euthanasia for adults.
"It is not a question of imposing euthanasia on anyone ... But of allowing a child not to agonise in pain," said Socialist MP Karine Lalieux ahead of the vote.
"The right to life and death cannot be restricted to adults," said liberal MP Daniel Bacquelaine.
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Addressing controversy over the decision not to set an age restriction for "discernment", he said a child's "legal age isn't the same as mental age."
Socialist senator Philippe Mahoux, author of Belgium's historic 2002 "right to die" legislation and himself a doctor, had called for the law to be widened to minors so as to offer a legal framework for medics for helping children in pain die as a question of mercy, but outside the law.
The law offers the possibility of euthanasia to children "in a hopeless medical situation of constant and unbearable suffering that cannot be eased and which will cause death in the short-term".
Counselling by doctors and a psychiatrist or psychologist is required, as is parental approval.
Before its adoption by a huge majority in the Senate in December, the upper house consulted dozens of medical specialists, lawyers and interest groups.
But during public debate, religious leaders of all faiths argued that extending euthanasia to the young risked "trivialising" death.