As the parent comment says, the law applies to children, and has even applied to a nine year old.[0] I don't think anyone believes that a child that young can meaningfully consent to a decision as significant as ending their own life, so obviously in practice the decision will be made by the parents, who will be influenced by the difficulty of potentially having to support a disabled child, financially and emotionally.

Then there are questions of the judgement of medical professionals. A research paper found that "Almost nine out of ten [medical professionals] (89.1%) agreed that in the event of a serious (non‐lethal) neonatal condition, administering drugs with the explicit intention to end neonatal life was acceptable."[1] This suggests that doctors would approve the killing of newborns even without a terminal illness.

Beyond the issue of the law applying to children, there are several controversial cases involving adults without terminal illnesses. For example, a man serving a life sentence was allowed to end his life due to his feeling of "anguish"[2], despite the country not allowing the death penalty for criminals.

Another adult was given euthanasia due to their opinion about themselves after a sex change operation[3] (which was presumably also approved by medical professionals).

Finally, a man requested euthanasia (which has been initially accepted) as a result of his dissatisfaction with his sexuality.[4]

[0] https://allthatsinteresting.com/euthanasia-in-belgium

[1] https://obgyn.onlinelibrary.wiley.com/doi/abs/10.1111/aogs.1...

[2] https://www.bbc.co.uk/news/world-europe-29209459

[3] https://www.telegraph.co.uk/news/worldnews/europe/belgium/10...

[4] https://www.bbc.com/news/world-europe-36489090