There are at least a couple of factors I see as relevant: choice, responsibility, and the notion of giving them a chance to live.
Children, necessarily, have much of their life controlled for them. They are not allowed to make a lot of important choices for themselves, whether they want to or not. So, it is important for those making choices for them to make the right ones, to justify not allowing them that control. I'm not sure I'm quite articulating the concept here, but...
It is the explicit social, legal, and moral obligation of parents to appropriate...
The thing I wonder is why humanity didn't insist that the superhappies refrained from acting on humanity until they had a better understanding of us. They made a snap judgement, that was obviously incomplete given what fraction of humanity opted for suicide under their plan--given more time, they likely could have come up with a plan that would reach their desired aims (not being made unhappy by humanity) with a minimum of distress to all parties...