And Damian Wayne, who first appears around age eight and attempts to murder the current Robin so he can take his place. A result of being raised by crazy ninjas, and also possibly the combination of Al Ghul and Wayne genes. Living with Bruce helped somewhat, but it's actually Dick Grayson who's really helped him into a solid HeelFace Turn.
Talia accepted this and started to work on a replacement, eerily telling Damian "He'll be ten years younger than you when he's born" So, taking the above into account, when Damian hits 18, there's going to be trouble (or sooner, since there might be a Plot-Relevant Age-Up for the clone).
Crossed has a few of these. The first encountered are the Kindergarteners in Volume 1, who are uninfected, but still set traps for people who are passing through the town where they're holed up.
Any children who become or are born as Crossed are creepy by default, including Patrick in Volume 1 and the (mostly unseen) children of the breeding Crossed in the +100 arc.
A particularly disturbing possibility gets brought up when discussing rumors about the "Super Crossed", humans who turn but retain some of their humanity due to brain abnormalities (or in one case, being so evil that turning really didn't change much). Apparently, one of them living in Montreal is a child who had autism before C-Day.
Ariel Chylde in Darkchylde isn't stoic and her voice isn't a harsh monotone. Adults find her creepy because at roughly 12 years old she's smarter than most of them and she reacts to everything with open hostility and condescension.
Emily the Strange, a 13-year-old goth girl with her own vocabulary of swear words, prefers rainy days and night time to daylight, and has an endless list of interests and hobbies that range from the silly to borderline gruesome. And the people she gets along with the best? Her four pet cats.
In the Neil Gaiman iteration of The Eternals, the childlike Eternal Sprite is reimagined as a Teen Idolwho also deliberately depowered the Eternals and wiped their memories, and who is aiding the Deviants in their efforts to awaken the Dreaming Celestial, all because the Celestials made him incapable of physically aging past age 11.
Fables has recently introduced us to the Literals and the Genres. Among the Genres, Horror is represented by a little blond girl in a pink dress.
Her older brother, Franklin, can also be this every once in a while. While he isn't a supergenius, he is instead a Physical God with the power to rewrite existence. And while he mostly acts innocent, he sometimes hints that he knows more than he lets on.
Grendel has Stacy Palumbo, the adoptive daughter of Hunter Rose, who eventually learns that he murdered her real parents and thus manipulates him into a final confrontation with Argent, in a plan that includes murdering her governess to prevent her from interfering.
Since childhood Joker's Daughter has been unusual. She was delusional, made a mobile out of knifes and corsets out of barbed wire, and killed her pet bird. As she grew older she began enjoying pain and Self Harming.
Innocence (a.k.a. the Child) is part of a malevolent and extremely powerful living tarot arcanum called The Basanos in the Lucifer comic book. She takes the form of a young, barefoot blonde girl and appears most often as their spokesperson. Her power is the ability to make people "see the world with fresh eyes" - that is, in a harsh light without illusions, delusions, or rose-colored glasses. As an added bonus, this power is exhibited as she walks down a seedy avenue in Amsterdam's Red Light district, causing everyone from hookers to cops to street people to come subtly repulsed at what they're doing.
Mike of Plutona is a lowkey example. He plays his game boy constantly, almost never speaks, and is easily recruited to defiling a corpse.
Rachel Rising, an Urban Fantasy/horror comic, has Zoe, a cute little 10 year old who carries out murders with chilling skill and no shred of remorse. It's hard to say if The Reveal that she's spent almost 50 years as the host of a Fallen Angel/demon that wants to destroy creation and eventually use Zoe to be the mother of his version of the Antichrist makes her more or less creepy.
Jordan Borchardt of Revival is as emotionally distant as any of the other revivers but it hits harder since she's a tween girl. Worse, she takes her mother's disappointment at not seeing God during her death too literally and slices off her own eyelids. She is unique among revivers in that her soul has not only been separated, it's destroyed. This allows her to imprison someone else's soul within herself, Horrifying the Horror.
Klara Prast of the Runaways, normally a Moe with a powerful Green Thumb, was suddenly transformed into a creepy child after a particularly traumatic accident during the last arc. While in creepy-child mode, she damn near destroyed the Runaways' house by summoning a small forest worth of vines and dispatched an entire squad of heavily-armed men. The latter feat was made even more disturbing by the fact that she had a completely blank expression while doing it.
The Cuckoo from the Sandman arc "A Game of You," arch-enemy of the Narnia-like dream-fantasy "Land". Despite her name (and true nature), she mostly appears in the form of a young, pig-tailed, freckled blonde girl — in fact, the childhood form of Barbie, the arc's protagonist.
Carnage's host Cletus Kasady as a child from Spider-Man his favorite past time was watching president John F. Kennedy's assassination footage over and over, he murdered his grandmother by pushing her down the stairs, he would often attack his mother and killed her dog with a drill, he played mean jokes on the orphans at the orphanage such as setting off firecrackers and smoke bombs in their rooms, and he murdered the orphanage headmaster and burned down the building.
Little Keiko in Usagi Yojimbo. Her only relative was robbed and killed by bandits, and they were about to kill her too when Jei, the book's most stubbornly recurring villain, appeared, declared them to be "evil" and killed them. Keiko almost got the same treatment, but he randomly decided she wasn't evil. She's been traveling with him since then, calling him "uncle", and her ability to remain cheerful through massive bloodbaths is chilling. What's also chilling that if this is a result of Jei's evil spirit suppressing her emotional development, the resulting emotional whiplash that is bound to happen if and when that suppression is lifted might likely traumatize her into insanity.
In Violine, some characters consider Violine is this because of her staring at them intently (she does this to read their minds).
Gail Simone's run on Wonder Woman had the Crows, five immortal beings disguised as schoolchildren who brew up paranoia and hatred in D.C. both naturally (through the distribution of hate literature and phony terrorist threats) and supernaturally (through a Hate Plague) as revenge against Diana for the death of their father, Ares.
This is what Kiden Nixon thinks about X-23 in NYX. Actually, everyone in the series finds her unnerving.
The X-Wing Rogue Squadron arc "The Warrior Princess" has Plourr flash back to her brother, Harran, when they were both children. Very definitely Royally Screwed Up, he was sociopathic, and the flashback shows him torturing an animal. Then Darth Vader, on a visit, used mental manipulation to transform his hot-blooded sadism into something cold and calculating. When a faction of nobles slaughtered Harran's family and one sister escaped, he tried to stop her so he could watch her die. She killed him and left his body to be eaten by scavengers. That's how she knows that the revolutionary calling himself Harran can't be her brother.