Generally speaking there is no reason why child euthanasia shouldn't be a thing. It's really stupid if you have to wait until your 18th birthday to be willingly euthanized.

As for who should have the authority, I think all 3 parties (child, parent, and doctor/hospital) should agree with the decision before proceeding, since those are all of the parties involved.

People doing strikes against this honestly have no argument to give. If someone wants to leave this world and their relatives consent to it, who's to say that it's wrong? People strike for the stupidest shit ever these days. If you love life then go ahead and treasure people around you, don't shove your morals down other people's throats.