Since I suggested the ending to David, right down to the candles, I
suppose I'll take the rap...but the question you're raising isn't the issue.
There are only two possible results: the kid lives, or the kid dies, there
ain't much in-between. You ask, "Who on earth is going to side with people
who kill their own child?" The audience isn't being asked to *side* with
anyone, there IS no easy solution, and no one is 100% in the right.
There is a wonderful short story, which we adapted for Twilight Zone,
called "The Cold Equations," where a small shuttle is going from point A to
point B. There is enough fuel for the shuttle, and one pilot, and no more.
The ship is bringing medicine to save 500 colonists. A young girl has stowed
away on the ship to see her brother. She's discovered. If the pilot does
nothing, the ship won't arrive, and he and the girl will die, and the
colonists will die. If he sacrifices himself, she won't be able/won't know
how to guide the ship to its destination. The only way out is to ask her to
enter the airlock so he can space her and continue the mission. And that's
what happens. You can't argue with math.
Sometimes, there are no-win scenarios. And what matters then is how your
characters react, what they do and say, and how it affects them. That,
really, was the thrust of the episode. And to go back to your question, "Who
on earth is going to side...."
The operative word in your question is "Earth." No, no human is going to
side with them (although I'd point out in the Bible that there is the story of
Abraham, who was quite willing to murder his own son at god's request).
They're not humans. They have a wholly different mindset, cultural background
and belief system. People ask for ALIEN aliens, then judge them by human
standards, and feel it's wrong if they don't behave like humans. These
didn't. That's who and what they are. If humans side with them, or accept
them, doesn't enter into it.