The Twilight Zone (1959) episode "It's a Good Life" features the cruel, sadistic, and omnipotent Anthony Freemont, age six. The episode basically shows what would happen if all of the selfish whims of a child were able to be fulfilled due to the child happening to have been born with incredibly strong, telekinetic powers. Anyone who tries to defy him gets killed, or worse sent to some place Anthony refers to as "the cornfield" where they're trapped for all eternity.
On Lincoln Heights, when Lizzie gets a basketball scholarship to an upper class boarding school the students initially appear to be welcoming, but it later turns out that they are racist, manipulative elitists who couldn't care less about her as a person and only care about her basketball skills. Basically, they expect her to act like "a girl from the hood." And the adults aren't much better.
The main character of Pushing Daisies was picked on at Boarding School, because of his introversion and his tendency not to retaliate. And when he did retaliate, the bullies waited until he was alone and then beat him up.
A major factor in Warren Mears growing up to be a stone cold, woman hating, megalomaniacal PSYCHOPATH.
Simon on FlashForward (2009) was bullied as a child growing up in Canada because of his accent and for being a Teen Genius. Until one day, when he filled his lunchbox with rocks and beat the crap out of the bullies with it.
Midsomer Murders likes this one; at least three episodes have children as murderers.
The Law & Order episode "Killerz" involves a ten year old girl who casually murders a six year old boy for the fun of it, and receives a slap on the wrist for it. During her psychological evaluation, she casually admits to poisoning the neighbors pets, of enjoying having killed someone, and of having plans to kill again.
Dr. Emile Skoda: "She's a Serial Killer. We just caught her early."
What's equally sad is the likelihood that the girl is a sociopath because of abuse she herself suffered — her father's in jail and she has tolerate her promiscuous mother bringing numerous lovers home and not even bothering to close her bedroom door when she has sex with them — it is implied that any number of these men has made advances to her as well. Is it any wonder that she's completely screwed up with a hatred of all things male?
Law & Order: Special Victims Unit had an example of a very young-looking thirteen-year-old date-rapist who was pushing his victim to get an abortion; his secret lovechild half-brother who had a crush on the victim killed him when it seemed like he was going to take matters into his own hands. There's also the two fourteen-year-old girls who almost succeed in pinning the murder of an eight-year-old on a retarded neighbor — actually one girl was "normal" and the other was the psycho/sociopath who tortured the kid because it was funny and strangled him because he was gonna tell.
An episode of The Office had a gang of kids attack Pam and Andy, later when confronted and asked why they assaulted them and threw things at them the leader replies because Pam was fat and they thought Andy was gay.