Yes, you can. Most Stephen King's books have children in them. IT is half narrated by childrens' point of view.
A child is innocent and generally untouched by evil, so it is a very good and interesting character for a weird tale. It forces the adult reader to look into dark themes with a "clean" or detached perspective, so an adult reader can be engaged and be interested. On the other hand, it reinforces the dark concepts, by showing them to a younger and weaker person, thus raising sensations of horrors in the reader.
What you want to avoid is making it gratuitous, such as having a kid suffer horrors just to make your story morbid. That is a meaningless choice, and I would avoid it - unless you want to write dark pulp pornography (which I would also not recommend, but that's just me).
Generally speaking, writing with the POV of a kid is more difficult: it requires good taste, intelligence and common sense, and a strong empathy with your characters. But it is not "forbidden" or ill-advised, at all.