As someone who has a relative who went through this at Christmas...
... my personal opinion is that if the quality of life is so poor that it makes your insides hurt with sadness when you see your loved one in such a condition, it is, of course, far kinder to bring an end to it.
In our case, the doctors were excellent and explained everything very clearly. Some members of the family were unsure initially, but the medical team kind of 'made' the decision which was just as it should be.
With a child involved... i dunno. Couldn't comment.