Leave a lot of it up to the imagination. That’s a pretty good rule of thumb for scary scenes in any story, because what you can imagine is often a lot more terrifying than words on a page.
Apart from that, rely a lot more on subtext. That is, layer the story with things that a younger kid might notice but that an adult would.
The Dementors from Harry Potter are a really good example of that. For younger kids, their appearance and the fact that they make you feel icky is scary enough. But they become scarier once you get older and you realize the implications of them being able to make you relive your worst memories – or even suck our your soul.
This is a helpful point! It’s important to know what your readers’ limits are. I was terrified of the Dementors when I was little, but having depression and just growing up in general definitely changed my perspective.
If you can make it visually spooky for children but have an effect on adults, then you’re golden.
Try thinking about what scares you–more than just being afraid of the dark. What do you value? Who do you love? What are you afraid of losing?
These are the things we think about when we get older. Being afraid of heights is not the same as being afraid of losing the people and things that matter to us most.