Replacing a child is impossible, it seems to me, because of what it is, or means, to have a child. Parents tend to love their children no matter what they are like, but what they love is those particular children. If they die then they cannot be replaced. Nothing that means anything to you (except as a mere means), I would think, can be replaced. Although something else might take its place. That is, you can love one cat and then later love another, but you cannot transfer the love you had for the first cat to the second one. That love either dies or stays with its object. We love individuals, not whatever happens to fall under certain descriptions. We don't love, that is, under a description (even if we would love whoever or whatever happened to fall under the description 'my child', 'my pet', or whatever). (I feel as though I'm failing to state the obvious here, but that's what I'm trying to do.)
Gaita couldn't shoot any rabbits that day, or at that moment on that day, because, to put it perhaps too crudely, the mood was all wrong. He was too happy, too in awe of nature to feel like killing any of its members.
Orwell's mood, too, was wrong for killing, but not morally wrong. He saw the man under the wrong aspect, under the wrong concept (fellow trouser-wearer, fellow man, something like that, rather than Fascist or enemy or threat), in the wrong mood to feel like shooting at him.
Each of these cases is related to morality, involving love, awe, and empathy respectively. But the last two aren't exactly generalizable or convertible to any principle, and the first one is more like a logical impossibility than a moral one. Although the logic of love is not wholly distinct from our psychology or morals.
Gaita wonders whether these cases of impossibility:
like the impossibility that we should consign our dead to the rubbish collection or that we should routinely number rather than name our children, [...] are impossibilities that structure and are structured by that part of the realm of meaning in which morality is embedded.The impossible acts are unthinkable, that is to say, in an important way. I've been starting to think in terms of something like ethical hinge-propositions, and what Gaita says here might seem to fit that way of thinking. But it has its dangers, such as the danger of leading us to think of morality as an isolable structure with its own scaffolding or foundation, and the danger of thinking of these foundations as more real than they are. I might explore this idea more though.