Pa Kent can not mean to be callous and still manage to be so. Giving even hesitant advice to a child that maybe they should let other children die is pretty much the definition of callous. Everyone has their own opinion on that scene of course, but for my money I would have preferred a slightly different approach. If the intent of the lines are to show that Pa does not have all the answers, and that he worries about his son, (and assuming I have to stick to a similar amount of screen time) I'd replace this:
"Should I have just let them die?"
"I don't know, maybe."
With something like this:
"Should I have just let them die?"
"Of course not, but...I don't know."