Sounds all-around reasonable what the Belgians are doing. Criteria that need to be met:
1. Parents, doctors and psychologists must agree.
2. Child must have been capable of rational thinking when making the call.
3. Child must be in the final stage of an uncureable disease.
Really, this is something that every country should have.
Indeed, and all of these criteria are taken very seriously. It's a lot harder to be euthanised as a child than as an adult.
That's why it took two years after the law was passed, for the first child to be euthanised: https://www.nieuwsblad.be/cnt/dmf20160916_02472929
And about the age of consent: they actually recently lowered it to fourteen years, because otherwise 14 and 15 year olds weren't even legally allowed to kiss with eachother. However, the age difference cannot be more than 5 years, so a 14 year old can't have sex with a 20+ year old.