MN, it seems you keep making the same flaw, but I love you, man!
Survival of the fittest would be if they all fought against each other, until one came out the winner. Now, keep this in mind, throughout the post. "abandon their morals" When you say they are an exception because they abandon their morals under such extraordinary circumstances, I want you to think about what you said. "their morals"!!! That's key!
People make mistakes all the time. I get frustrated and blurt out a word now and then that I wish I didn't. It rarely happens, but it does.
What is still key here, even if you want to make it a minor side plot, is the reluctance to leave the children behind. It doesn't matter what they did, because people fail to do what they should do every day! That doesn't have anything to do with "morals". What does have to do with morals is their impressed feeling that this is only something to do as a last-ditch effort for some to survive. They held out as long as they could. They struggled with the decision. If it were not morally wrong for them, why would this be so difficult to do? The earlier they ditched them, the greater chance of making it to safety. The longer they postponed ditching them, the less likely they would make it to safety.
So they held out until all
reluctantly agreed that they had to do it.
Absolute Morals are not exceptions people will make. Their actions aren't the moral issue. It is the acknowledgment that something is amiss with this decision. Their circumstances were bad and getting worse, but they still struggled over it, because they wanted to avoid doing it. THEY WANTED TO AVOID DOING SOMETHING THAT FEELS WRONG. <--- There is your Absolute Moral again!!
I hope you understand what I'm saying, because you aren't focusing on the thing you should be focusing on when deeming whether an Absolute Morality was in play.