You can go either way, some of the most popular fiction ever is directly targeted at teens, and then you have extremely successful modern fiction like Game of Thrones.

It sounds like you’re targeting a demographic like the GoT audience which isn’t a bad thing. If you can actually write an ending you could end up with a more popular work…

I think everyone here has good points, but I want to introduce a counterpoint to it as well.

Some of the most beloved "children's" or "teen" series of all time do not pull their punches on that. Two examples that come to mind are Animorphs, and A Series Of Unfortunate Events. ASOUE doesn't go into gritty detail about it, but the entire series is an extended chain of traumas that these children go through. Kids loved it.

Animorphs sometimes DOES go into the gritty detail. There are things in those books that, looking back on it as an adult, I think might not have been entirely appropriate for a children's series. But kids like it, because it takes them seriously. Most children's series are whitewashed to hell and back for reasons that seem to make sense, but also make for a boring story that you can often accurately predict the beats of by the second chapter. Animorphs doesn't pull punches though, and it touches on the experience of the human condition and it expects kids to relate to that. Which they can, they're humans too, and they live real lives like the rest of us. Most children's book series never give their audience that kind of respect, and when I read them when I was younger it was very refreshing to find a series that did.

You wouldn't expect, say, the political violence, terrorism and unending war of Star Wars to be something that kids would latch on to, but they do. Kids love Star Wars. A lot of that is probably because it's an ostensibly "family friendly" story but in truth it wasn't actually written for them, it was written for their parents. It's a grown up story.

Now, all that to say, I think there's nothing wrong with writing a gritty, grown up children's story, and the target audience is likely to appreciate that you've done so. But, that said, there is a balance to be struck. Kids know what sex and violence are, but they probably shouldn't be a core focus. If you're writing for a younger audience you don't have to dance around these like they don't exist, but I wouldn't write them in compelling detail. They don't need to read a detailed 3 page description of sexual violence, but they are capable of understanding what that is and what it means if it is a necessary plot point.

A lot of children's authors just strip all the interesting parts out of their story in order to remain "appropriate" to their audience or to appeal to what they think the audience wants. But largely, kids just want to be taken seriously and treated like real human beings. Showing them a little respect and trusting them to pick up on what you're putting down, like you would with an adult reader, can go a long way towards making a beloved story.

Also this is a great site to explore ideas for your stories if you haven't seen it: https://worldbuilding.stackexchange.com/

I read Stephen King novels as a preteen and teen. So I don't think I can offer solid advice.

Or maybe, just don't do more than Stephen King, and you should be fine. I know that's not saying much.

Write what you would want to read so it will be authentic and like-minded people will also like it.

Skip the dark stuff, we have enough of that in the real world. We are reading fantasy to escape and spark.our imagination. You can use sophisticated language and idea and have deep characters without being dark.