When you write for children ages 3 - 6, the poem or story should entertain and delight. Fictional pieces should not frighten very young children, even if the piece is told in jest. Parents may appreciate the humor, but a child might get upset or be confused.
For instance, I received a story for Kid's Imagination Train about a young person who had invited a group of animals over for a play date. This is a cute idea and the writer was off to a good start. The piece was lively and amusing. But towards the end of the story, the mood got dark. The more ferocious animals began to eye the other harmless animals. Do you see where this is going? Yep, the vicious animals ate the defenseless animals—bones, fur, scales and wings and all.
If handled delicately, dark humor may work for young kids. However, this kind of humor is usually better suited for an older audience. If you want to write a piece like this, you should find a publication whose audience ranges from 8 - 12 years old. You can find out if an editor publishes this kind of humor by reading some back issues of the magazine. And, you can query the editor to find out if she would be interested in such a story before you submit it.
Always remember your audience when you are writing for kids. If you want to write for very young children, keep the writing lighthearted and playful. But if you want to create scary, then make sure these kinds of stories end up in the hands of older readers.