Mr. Mahoux said in an interview before the vote that euthanasia for terminally ill children was already practiced on occasion in some Belgian hospitals and that the law would not lead to a surge in medically accelerated death among sick children but would save doctors from potential criminal prosecution.

The amended law extending the “right to die” to children mandates that euthanasia can be carried out only at the demand of a patient and that such a request be “voluntary, considered and repeated and not the result of external pressure.”

Unlike adults, children would not be allowed to choose death on the grounds of “psychological suffering” but only when there was no hope of recovery from an illness that involves extreme physical pain. Parents must give their approval in writing.

Religious groups, however, view Belgium’s efforts to extend its already contentious 2002 law to children as a dangerous erosion of moral barriers protecting the sanctity of life. “We mark out opposition to this extension and express our trepidation in the face of the risk of a growing trivialization of such a grave reality,” the leaders of Belgium’s Christian, Muslim and Jewish communities said in a statement.

During a debate in the Senate on Thursday that included some angry exchanges, members of the Christian Democrats denounced the changes as open to abuse and fraught with peril.

Els Van Hoof, a Christian Democrat from Belgium’s Dutch-speaking community, argued that paying more attention to relieving the pain of patients instead of allowing doctors to legally kill them would “allow both old and young to die with dignity.” A 10-year-old, she said, is not in a position to make a life-or-death decision “in an autonomous manner” and will invariably be vulnerable to pressure.

Mr. Mahoux, who is a former surgeon, said “it is particularly painful to touch on the issue of death” for children, “but what is scandalous, in the primary meaning of the term, is first of all the illness of minors, incurable illness, their unappeasable suffering, and not their will to want and to be able to put an end to it.”