It really depends on who you’re writing for and what you’re comfortable writing…. personally, I don’t mind dark novels as long as they have good themes behind them(*looks at my own novel* Yeah, I don’t really get to say not to write dark novels…. XD), and don’t dwell on it too much.
But then there’s the actual execution of the death–is it just plot fodder, or will it actually affect the story in a meaningful way? Let’s take the end of the Maze Runner for example. I read that book at maybe…. 12? 13? Something like that, and honestly, I thought it was pretty cliche and boring… and then you got to the end. The characters had just escaped the maze, figured out some important things, and then in like the last few pages there was a very graphic description of gore/death(for my 12yro mind at least) of what(if I’m remembering correctly) was the lab that was in charge of the maze beforehand. It was a rather dark turn for the novel that had seemed like it was going to have an okay end, but….. it still was rather flat, to me. It didn’t really shock me or make me sad–it was just–oh, they died. Okay.
I’m not a particularly emotional person, and I don’t get attached to characters easily, but you’d think I’d think something of this scene since it was obviously meant to be shocking. But it wasn’t because it was plot fodder. “This is an angsty teen dystopian novel, so let’s throw in a scene where the characters find the lab that was controlling them the whole time was destroyed and everyone there died… because PLOT.” I’ve never gone back to finish the series, because the first book bored me so much.
Back to the point. What is the theme of your novel? Is it the downfall of man without God? Is it the loss of innocent life? This can make or break a “sacrificial” scene. If your theme is “bad things happen to good people”, that sounds like an okay theme(and can be totally pulled off)…. but it might not work because it’s just that. Any death you put in there is just going to seem to be a cheap plot device to push the theme on readers, unless you really get it right.
So let’s say your theme is the fall of man without God. You kill off this little girl, but where you go from there afterwards is very important also. You can generally take it in one of two ways–the MC falling even farther away from God in despair, or him coming to accept this death and living on to try and prevent things like this from happening again. Where you take it depends only on you, but it’s very important so think it through carefully.
Now let’s assume you don’t kill this little girl. Personally….. I think the other way around would be more unpredictable, but it’s also a pretty sensitive topic and if you don’t feel comfortable writing it, I wouldn’t. At least not without thinking about it and why it’s a good thing to do carefully. But if you don’t kill her, will this affect the plot? Will it contribute to some major plot areas, like the worldview of the MC changing? There’s still the possibility of a redemption arc even without killing her off–you could have her injured, or kidnapped and used against the MC, or you could have him feel guilty for pulling her into whatever the conflict of the novel is and leave her behind, only to realize that he’s hurting her more that way than if he took her with. You could have her injured and him leave her to heal them go off to hunt down whoever injured her and exact his revenge, only to realize that’s not what she would want(a little cliche, but if you pull it off it can be rather powerful).
Keep in mind–children are one of the most powerful motivations of any character, no matter what age.
"A hard heart is no infallible protection against a soft head."
- C. S. Lewis