Adventure Time. Past the post-apocalypticbackstory and severe psychological issues of many of the cast, the show is full of sexual innuendo, thinly veiled references to rape, murder, abuse, suicide, and genocide, and some viciously brutal violence. There are episodes dealing with the pain of love lost, the complications of love found, raising children, reconciling with parents, dealing with a mentally ill loved one, abusive relationships, Trans Sexuality and more, all wrapped up in the silly adventures of a boy and his dog. It's kind of amazing how many serious, mature, or just horrifying themes that are slipped into the show without technically moving past what is appropriate for children.
Courage the Cowardly Dog is packed to the gills with scary scenes, like the screamer girl from "Courage in the Big Stinkin' City" and the blue...something from "Perfect". It ran for four years and got canceled for being too scary, which isn't too surprising.
The Grim Adventures of Billy & Mandy: Not only because the main premise is about two kids that became friends with Death Itself, but also because the show depicts a series of grotesque situations quite unusual for a children's show.
Even better is Regular Show. More sex jokes, frequent lethal use of weapons and mild profanities ("crap", "sucks", even "pissed") then you can shake a yardstick at. Justified, as Regular Show is based on two short films J.G. Quintel made in animation school called "2 in the AM-PM" and "The Naive Man from Lolliland." While "The Naive Man from Lolliland" is safe for family viewing (the one use of the word "hell" wouldn't phase most viewers), "2 in the AM-PM" isn't — at least by Cartoon Network's already selective standards.
Robotomy: Excessive violence, a lot of Comedic Sociopathy, some sexual innuendo, some mild swearing (mostly words like "crap," "sucks" or "screwed"). Justified, as one of the show creators worked on Superjail and it has the look and feel of a Superjail spin-off or companion show.
Scooby-Doo! Mystery Incorporated, which is pretty much Scooby Doo if it wasn't so campy and 1960s. Despite being rated TV-Y7-FV, the show includes moments of death, extrme violence, Black Comedy and the season one Big Bad being revealed to have kidnapped Fred from his birth parents and threatening to harm him if they ever came back for him; to make things crazier, the Bigger Bad is an Eldritch Abomination that the gang had to kill in order to save the Universe. Not to mention an abnormally high amount of Stuff Blowing Up very realistically, up to the point where near the end of the series, roughly half of Crystal Cove is gone!
Teen Titans was pretty kid-friendly overall, but it did have moments of this at times with storylines involving the end of the world, Blackmail when Slade threatens to kill the Teen Titans unless Robin becomes his apprentice, and a few instances ofMind Rape. Not to mention Raven's demonic heritage would make some religious parents and viewers scared.
Time Squad: Here's an apt description of the show: on the outside, it was a funny, unassuming edutainment cartoon (that was more entertainment than education) about an orphaned history whiz taken in by a Time Cop and his Robot Buddy to the future where, each episode, they go back in time to fix history. On the inside, it had more Ho Yay than the original Star Trek, got away with more adult jokes than Rocko's Modern Life, played up the Hilariously Abusive Childhood trope for laughs more than The Simpsons and South Park combined, and seemed to indulge in more homoerotic subtext than anything Oscar Wilde has written. Is it any wonder that Cartoon Network aired it at five in the morning during its final years?
The subplot in "Beneath" heavily implies that the mother of one of Jaime Reyes' friends is being physically abused by her boyfriend. The same episode also reveals that Queen Bee is essentially running a child-trafficking ring, where innocent teenagers are kidnapped and sold to aliens who use them for experimentation.
One of the protagonists' favorite tactic is Mind Rape. And it is played out for maximum shock value.
Avatar The Last Airbender is a show about a war that has lasted for a hundred years, tearing apart families and nations and was begun with the genocide of an entire people. The protagonist, the 12-year-old last survivor of said people, assembles a team of Child Soldiers and trains to overthrow the Evil Overlord (who happens to be a shockingly abusive parent) that seeks to subjugate or kill anyone who isn't Fire Nation.
The "Southern Raiders" episode in general, for its portrayal of murder, revenge and forgiveness. It doesn't go the way one might expect.
The season finale is about preventing said Evil Overlord, who has gained a massive two-day power boost, from immolating an entire continent.
To continue the tradition, The Legend Of Korra revolves around Fantastic Racism, the terrorists that form in response to it, and the totalitarian oppression in response to that. The whole plot is ultimately kicked off because of child abuse, and ends in a Murder Suicide. It's a wonder how this show is rated Y7-FV.
Not to mention that the method of Debending looks eerily similar to public executions.
And that's being subtle and mature compared to Korra straight up Jack Bauer-ing a guy in Book 2 by sticking his head in Naga's mouth and threatening to turn him into her afternoon snack.
Book 2 also includes two of the most vicious cameos ever committed to celluloid. That nice professor who refused to leave Wan Shi Tong's library? He's still there... as a dessicated corpse. And Admiral Zhao died, right? Not exactly... his soul is still around, trapped in a spirit prison and driven insane by his own worst memories.
In general, the show has a tendency to play disturbing imagery completely straight. When Korra's limbs are temporarily erased by getting struck by bending in spirit form or the spirit of darkness forces himself out of Unalaq's mouth,latches onto Korra's face, and rips the spirit of light out of Korra's eyes and throat, it's not an attempt at gross-out humor in the slightest, just kind of horrifying.
Invader Zim was more grotesque than most of Nick's other works, featured things like children summoning demons and a disturbing amount of body horror (including one kid getting his eyes plucked out), and was created by a man who wrote a comic series about a homicidal maniac.
Parodied at the end of the very first Treehouse of Horror episode. Homer thinks he cannot sleep because the stories told on the show were scary, but Marge says that they are just "kids stories", when clearly, some of them weren't intended for kids in real life.
Freds Head goes one step further than the above shows with things like swearing, handling of mature situations outright, and other non-kid friendly material. Unsurprisingly, the show wasn't renewed for a second season.