Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Arthur
Being a slice-of-life cartoon (albeit using anthropomorphic animals) about the lives of young children, Arthur had a few moments that did a good job of depicting the dark sides of children's overactive imaginations.
In that same song, if you were to pause it after the Brain drinks the formula at the beginning at the right time, his head looks... really, really weird, to say the least.
For a kid's show, the series can be overripe with some pretty traumatizing examples, particularly "Night Fright", which deals with Binky confronting some of his long repressed nightmares. The monstrous hamburgers, attacking fries, and giant lips on a billboard are freaky enough, but probably the most terrifying moment in the episode was where Binky dreams he's trapped inside his nightlight, while his friends gather around to mock his nightmarish plight. The nightlight is shaped like a clown, by the way.
The giant purple orange, also a part of one of Binky's dreams from the episode "Rhyme For Your Life."
The Gainax Ending of The Boy Who Cried Comet would've probably caused nightmares with a few of the fans as well.
The episode "Arthur's Lost Library Book." Arthur loses a library book and has a series of disturbing nightmares as a result of his guilt, including one where a Ms. Turner (the librarian) forcibly pries him out of his bed via a chain and drags him to the library to be punished for losing the book.
Ms. Turner: Next time, don't make me use the crank!
The episode "Locked In The Library" has plenty of disturbing imagery from Arthur's fear when he and Francine are trapped overnight in the library.
Hell, imagine being shut in without food or contact for possibly two days.
Arthur: I just realized something.
Arthur: Today's Saturday, which means...the library will be closed until...
There is an episode where Arthur gets lost while taking the wrong bus across town. Because of a story his friends told him about the bus driver, he has a nightmare where the bus driver refuses to let him off the bus and randomly morphs into a disgusting-looking squid monster.
"Francine's Pilfered Paper" is so forceful in the delivery of its message that it seems more like a television adaptation of The Tell-Tale Heart. Plagiarism is a serious offense, but the production team tries so hard to scare us straight that it winds up being scary.
The scene in Kate's debut episode where she cries at the top of her lungs, keeping D.W. awake. The closeup of her dead eyes wide open, mixed with Kate's crying (they even add reverb) is bone-chilling.
There's a similar scene in "Arthur Babysits" involving Rubella and the Tibble twins.
In "Arthur's Knee" Brain informs Arthur about all the possible complications that can arise from his injury. What he lists is pretty disturbing. It includes tetanus, encephalitis, staphylococcus, conjunctivitis, and other such unpleasantries. Even D.W. was freaked out.
Every now and then, there's a song that will play that usually plays when something bad happens to one of the cast. It's ominous and chilling and certainly will creep some people out every time they hear it. Starts at about 3:41.
Before the title card for "I Wanna Hold Your Hand", Binky glares down the audience when he thinks they're making fun of him. It's not a pleasant sight.
The spinach heads scene from "I'd Rather Read It Myself". The way their skin turns green and their heads slowly turn into giant spinach leaves and the creepy zombie like *bleahs* can scare a lot of kids from eating their vegetables.
There is an episode where the Tibble Twins watch a TV show about robot superheroes. They become completely obsessed with it, up to and including wearing homemade costumes to preschool and acting out scenes from the show. This takes a turn for the terrifying when the Tibbles chase after D.W. (who they are pretending is a bad guy) and throw a swing into her face. D.W. goes down into a heap on the grass, crying her eyes out, with her face out of view. Emily then rushes over in alarm to D.W. only to recoil in horror and scream, "BLOOD! I SEE BLOOD! MISS MORGAN!" And then after your seven-year-old mind is reeling from that, it cuts to D.W. at the hospital with a lip full of stitches.
In the episode "D.W Aims High" when D.W. dreams of being an astronaut, the Tibbles scare her by telling her that there are skyscraper-sized Martian dinosaurs that eat their own species. They go on to tell her that the reason they call Mars the "red planet" is because of the Martian blood that glows at night - how's that for Cosmic Horror! Even though it's not true it's pretty scary for a kid to think about.
The intro to "How the Cookie Crumbles" features Arthur, Francine, Binky, and Brain as living cookies, and Muffy trying to eat her friends. Her apathy and the scene as Cookie!Arthur backs against a glass of milk make an unnerving segue into the episode.
In the episode "Arthur's Mystery Envelope", Arthur has a nightmare in which the envelope he hid in the kitchen (that he got from Mr. Haney but was afraid to give to his Mom) keeps growing every time he tries to hide it, until he finally gets his confused mother out of the house with him as it breaks out of the house and even towers over the city.
Not to mention D.W. popping out of the envelope and asking "Didn't you tell Mom yet?" (echoes)
"The Tales of Grotesquely Grim Bunny", in which Arthur gets scared by a horror comic. It's… unsettling, to say the least.
The episode "Fernkenstein" has Fern telling a rather frightening story about an umbrella-shaped monster known only as The Thing, with the rest of the cast becoming obsessively afraid of the imagery, and for good reason. "Horrified shrieks" and "satisfied slurps" are PRETTY gruesome ways to describe someone's death in a horror story, especially on a children's show.
Brain's nightmare about the world being overtaken by giant, cycloptic worm monsters.
One episode has Arthur and friends trying to make a James Hound fan film. While brainstorming for plot ideas, Buster comesup with the idea to have the Bond villain shoot a fish hypnotic ray to hypnotize the fish of the ocean to freaking EAT people!
What really sells it is the fishes' creepy voices as they chan "We must eat the land people, we must eat the land people." And then they actually try to get up onto the beach to get the humans! Jesus, Buster.