When a child shows up in an anime, most viewers expect that they'll be adorable and innocent - or maybe even annoying - but few expect them to be straight up villainous. Anime villains who are children, and they're all the more effective because like viewers, anime characters also don't expect that a big-eyed child carrying a teddy bear is going to be deadly.
Some of these evil anime children are remorseless killers who don't seem like they're ever going to change - that's why Hibana Daida is in prison despite being a seven-year-old girl. In other cases, they realize that what they'd rather side with the good guys - when 11-year-old Ken Ichijouji of Digimon 02 realizes that what he thought was a video game has real consequences, he immediately quits his role as the Digimon Emperor and begins protecting the Digital World he once sought to destroy.
Whether they eventually turn good or not, you definitely don't want to get on the bad side of these villainous kids.
Selim Bradley - Fullmetal Alchemist: Brotherhood
Though he appears to be an innocent young boy, Selim Bradley is actually the first homunculi created by Father. His true form is a monstrous shadow that he unleashes whenever he wants to kill a human - which is frequently, since he sees himself as superior to humans and delights in tormenting them. He's even willing to turn his ire on his own allies - he betrays his fellow homunculus Gluttony by eating him alive.
Despite his sadistic nature, Selim has grown attached to his human mother, and enjoys living like a normal human child. At the end of the series, he's able to leave his villainous past behind and live like a regular kid.
Kanna is one of Naraku's many child incarnations. At first, she acts solely as an emissary of his will, and attacks Inuyasha and his friends. Naraku felt quite sure that she would never develop her own will and betray him, and trusted her to give direct orders to his other incarnations, Kohaku and Kagura. She manipulates a village full of people to try and take possession of Kagome's soul, and gets into a life-or-death struggle against Inuyasha - which ends in her death. Just as she's about to pass away, she tells Kagome how to defeat Naraku - finally developing a will of her own in her last moments.
Mariko Kurama - Elfen Lied
In some ways, Mariko Kurama is no different from any other five-year-old girl. She wants to have fun and be close to her family. Unfortunately, those childish traits are overwhelmed by her time being experimented on by the Diclonius Research Institute, as well as her raging jealousy regarding her father's parental affection toward another Diclonius girl. She grows obsessed with killing anyone wo comes near her, be they human or Diclonius, and delights in harming others as she was once harmed.
Hänsel & Gretel - Black Lagoon
Hänsel and Gretel are a pair of murderous twins who were dumped in a Romanian orphanage. Though they are terrifying killers whose prowess cannot be ignored, their cruelty didn't come out of nowhere. As very young children, they were regularly forced to kill other children in order to avoid being beaten at the orphanage - and even worse, the two of them were forced to star in snuff films. These experiences caused them to believe that killing other people increased their own life spans. Though ultimately killed in order to stop their excessive killing, the protagonists of Black Lagoon can't help but feel some sympathy for these miserable twins.