The letter argues that the law "responds to no real demand" and that most medical teams caring for terminally ill children would recognize that none of their patients has made a spontaneous and voluntary demand for euthanasia.
Meanwhile, medical advances mean that effective palliative care is available and that children do not suffer as they approach death. Extending the "right to die" to minors will only add to the stress and pain of families at a difficult time, it said.
The letter also questions how any objective judgment can be made on a child's ability to understand what's at stake.
The political process has created a "false impression" that a change to the law is urgently needed -- but in reality "the situation in our country is far from being dramatic," the doctors say.
Others also question whether children have the capacity to take this most final of decisions for themselves.
Palliative nurse Sonja Develter, who specializes in end-of-life care for children, told CNN she is concerned that giving children a choice would mean they made decisions based on what they thought their families wanted to hear, and that it would be a terrible strain for children who may already feel they are a burden to their caregivers.
'If you leave, you leave forever'
Izabela Sacewicz has Huntington's disease, a neurological disease that drastically reduces life expectancy in children.
Eight years ago she was a bubbling, active child -- top of her class, according to her mother, Iwona. Having recently turned 18, she can't eat or walk without help. She finds it difficult to speak, but her mind is still her own.
In a painful exchange, her mother explains to her what euthanasia is, using the simplest terms she can think of.
"Euthanasia means if you are unwell, you are so unhappy that you don't want to stay here, you want to leave, to go high up to God," she says. "But if you leave, you leave forever."
Izabela listens, the strain showing on her face.
"Do you think it's good, or not good?" her mother asks.
"It's not good," she replies, the words barely audible.
Iwona says that with enough support, no parent would think of euthanasia -- and that Belgium's lawmakers should instead focus on providing better support for families caring for terminally ill children.
Supporters insist the measure is more a matter of principle than anything else -- and that only a small number of children will ever, in practice, ask to end their lives through euthanasia.
Under its strict guidelines, no doctor would be forced to carry out euthanasia against his or her will and the child would always have the option of palliative treatment.
A child psychologist or psychiatrist would have to examine the child to make sure he or she is capable of making the decision.
In the Netherlands, where children have been able to request euthanasia with parental consent since 2002, only five children have ever done so.