Morality is doing the most good for the most people. If a disable child ruins 2 parents lives and their oldest child live and provides no utilitarian value when the alternative is 3 productive people and killing the one child I would say that morally you're doing less damage over all by killing the severely disabled.

I want to reiterate I don't think that morality is always doing good I think it is doing the most good for the most people even if that means intentionally causing some badd things to happen to certain people, I do think the bar for doing that is very high. Ending a life that will never have happiness and doinf before it's had a chance to even begin isn't that extreme. so is far below this bar