Memorably subverts this in its second season. At one point Jack orders the execution of an innocent child while his terrorist father watches in order to trick him into revealing where a nuclear bomb is being hidden. Then after the terrorist is taken out of the room, it's revealed that the entire execution was elaborately staged and the boy is alive. On the other hand, villains can play it very straight, such as some of the fifth season's antagonists.
In season 5 Evelyn Martin and her 8 year-old daughter are killed offscreen.
Adam-12: Several episodes dealing with child abuse; the assault itself would never be seen on camera. The most memorable child abuse-themed episode is "He ... He Was Trying to Kill Me" (from the spring of 1969), where a 6-year-old girl lets on to juvenile protection that her 'daddy' hits her. He didn't. Little Charlie was trying to protect her mother.
Both Angelus and the torture expert vamp from 'In the Dark'. In fact, Angelus's first act as a vampire was to murder his young sister after she invited him into the house.
An early episode revolves around a young boy possessed by a demon. At one point the demon tries to kill the child by making him walk into traffic, which would have ended the demon's life as well. It turns out the reason behind his attempt on both their lives is that the child himself was more evil than the demon, so much so that the demon was willing to die just to be free of him.
The series premiere had a terrorist who kidnaps a girl, straps a bomb to her and plans to detonate it in an area frequented by children.
The second episode features the Everheart Cartel, a criminal organization that specializes in child-slavery.
Boardwalk Empire: Fan favorite Richard Harrow has a very sweet connection with Margaret's two young children and in season three, Jimmy's son Tommy...but he doesn't hesitate to offer to kill the entire D'Alessio family, including the mother and sisters, and does in fact shoot the youngest brother in the face. He's only 14 and is crying as he frantically attempts to load his own gun, before raising his hands, trying to surrender. Whoops.
The Brady Bunch: Humorously troped in "Bobby's Hero," where in a dream sequence, Jesse James shoots and kills Bobby's siblings (along with his parents and Alice) during a train robbery ... all to make the point that the famed outlaw was nothing more than a "mean, dirty killer."
Some gangsters draw a kid into their gang, having him kill a competing drug dealer as a hazing. They later kill this kid after being told by their druglord boss they weren't allowed to use kids anymore.
A more significant example comes when Brock is poisoned by Walter, in an attempt to make it look like Gus had poisoned him, so that Jesse would turn on Gus. It's a clear Kick the Dog moment as well as something of a Moral Event Horizon for Walter.
And later, in Season 5's "Dead Freight", Todd shoots a 10 year old boy who accidentally stumbles across their heist. The wildly diverging reactions to the killing between Jesse and Walt emphasize how far the latter has fallen: Jesse is completely traumatized while Walt puts a shallow mask of guilt over an obvious reality of indifference.
The Aryans get Skyler's and Jesse's cooperation by threatening to kill children. Considering all the other horrible things they had done, there is no doubt that they would do it.
Every thirty or so years the Mayor feeds newborn babies to a sewer-dwelling demon named Lurconis.
Oh Adam, you get up to such shenanigans in your first few minutes of life.
Drusilla: "What will your mummy sing when they find your body?"
Monroe and his group have no qualms attacking Bayarmaa even though she is carrying an infant.
The Charmed Ones come across a trio of children in "We All Scream For Ice Cream" that are actually the spawn of upper-level demons. Initially reluctant to hurt them, the children use their powers and the sisters fight back. Notably Prue uses her telekinesis to fling a little girl into a fence.
Prue: Oh, you need a time-out missy.
Criminal Minds: Certain criminals like Carl Buford in "Profiler Profiled" and Karl Ahnold in "The Fox" have no trouble hurting and killing children. Some even specifically target kids!
CSI NY: The guy who killed Lindsay's friends in her backstory.
In the Season Two finale, Lila kidnaps Cody and Astor - both young children - and locks them and Dexter in her loft before setting it on fire. Thankfully, they escape.
In the fourth season, it is shown that the Trinity Killer has no qualms about kidnapping ten-year-old boys, holding them prisoner, drugging them and then killing them by Buried Alive BURYING THEM IN WET CEMENT. WHICH THEN SETS. This is also why he went uncaught for so long. The killer's name, MO, all of it, was an accidental misnomer, because nobody ever knew about the FOURTH victims in each set representing himself.
The sixth season's villain proved he was more than willing to kill a child.
Forever Knight: Had LaCroix make a child vampire once and a killer in the present day part of that story trying to kill a girl who saw him kill someone.
Dear, loveable Mad Scientist Walter Bishop... experimented on children, occasionally frightening them to obtain results, and with often devastating consequences for them in adulthood; he also kidnapped his alternate-universe son and lied to him (for his own good, admittedly).
Game of Thrones: Jamie Lannister defenestrates note pushes out of a windowBram Stark, rendering him temporarily comatose and permanently crippled.
Good Times: Though little to none of the actual assaults were shown on-screen, the four-part 1977-1978 season opener, revolving around 10-year-old Penny Gordon (Janet Jackson's series acting debut) more than left no doubt she was struck. The vicious so-and-so who hit Penny was her own mother (Chip Fields), a single parent who took her frustrations out on the innocent girl. Several infamous scenes were shown, including Mrs. Gordon stalking her with a hot iron (the scene cuts before she places the iron on Penny) and Penny shrieking in pain because of a broken arm.
Heroes: Has an Anti-Hero example in Noah "HRG" Bennet, who is willing to shoot Molly Walker, whom the Company is using as a human superhero-tracking system, in order to keep them from finding his own daughter, Claire. He is at first prevented by Mohinder holding him at gunpoint, but only lowers his gun when his temporary partner, Matt, recognizes Molly as the girl he saved earlier in the series.
I, Claudius: Children get executed without so much as a blink. When reminded that it's against the law to execute a virgin, "Then make sure she's not a virgin when you kill her!"
14-year-old Lorretta is kidnapped and locked in the trunk of a car by a pedophile. In a later episode she is almost killed by Coover Bennett. A season later Dickie Bennett breaks into her house and it is clear he would hurt or even kill her to get the millions that Mags Bennett left Loretta.
In season 3 Robert Quarles carjacks a mother and her two sons. He uses the two boys as hostages to get Raylan to give up his guns and then drive them all to Noble's Holler. It is clear that once he gets what he is looking for, he will kill Raylan and the children.
Kaizoku Sentai Gokaiger: This is done in an extremely horrific fashion by Basco. In an attempt to get one of the Great Powers, he uses his Ranger summons to stop a badly hurt young boy from being taken to get medical attention, knowing full well the boy could die. He then one ups himself by lowering his gun to shoot the kid! And is smiling the entire time!
In fact, in that series, it was implied that most monsters that Rita had at her disposal would not cross this line, seeing as when she needed one that was willing to kidnap or hurt a child, she had to tell Finster to create one that was especially cruel. Apparently Eye-Guy and the Chunky Chicken fit the bill. (The second one was actually willing to kill Trini's younger sister after taking her hostage.)
Little House on the Prairie: Several episodes about child abuse. Although the actual assault was rarely seen, they were seen in a flashback in at least one episode (where Charles is helping reform a budding juvenile delinquent, and the boy — upon being given a present — snaps when he sees a shirt, identical in style to one that he was viciously assaulted in by his own father). In another episode, where Charles and Caroline are debating whether to adopt orphans James and Cassandra Cooper (whose biological parents were killed in a wagon accident), the orphans' temporary foster father whips James (off-camera) after being unfairly accused of stealing; his screams were heard as Cassandra was forced to watch. At least once, a teen-aged boy of 16 kicks and repeatedly punches a little girl less than half his age at recess — one that also earns Mary (who is 14) a punch in the mouth when she tries to stop it.
LOST: Sayid shoots a 12 year-old boy in the chest (Ben Linus in 1977), believing it to be for the greater good.
Maddigan's Quest: This seems to be the mark of a bad guy, in which it's asserted that the Nennog would kill Timon, Eden and baby Jewel without a second thought if they returned to their own time, and in Greentown, henchman Maska threatens to 'break' Jewel if Boomer doesn't return his bag. In Laketown, Timon tries to kill Jewel whilst under the Nennog's influence, but can't bring himself to do it (whether or not he would have done it if the Fantasia had failed is a moot point, though he certainly seems to think he would).
Masters of Horror: The show featured several villains who were so evil or monstrous that they would kill children.
"Incident On and Off a Mountain Road": A dessicated baby corpse is amongst Moonface's "trophies".
"Dreams in the Witch-House": The witch ritualistically sacrifices infants for unexplained reasons, forcing various helpless men to do so. At the end it's revealed that the house's walls contain corpses going back all the way to the 1600s.
Root would hurt a child. She would hurt anyone, actually.
There was an episode in which the POI was a 15 year-old girl who was believed to have been killed two years earlier in a hit on her parents (who weren't so lucky) and Reese had to protect her from being Killed Off for Real.
Carl Elias forced Reese to tell him the location of a rival Mob boss by locking him in a refrigerated truck with a baby. He claimed the baby was never in danger because he knew Reese would tell him what he needed to know, but even if he wholeheartedly believed that, the gambit alone stil qualifies him for the trope.
Neville - sort of. Danny technically is not a child anymore, but most characters treat him as such, because of his age, and probably also because he has demonstrated impulsiveness and rudeness and other negative traits common to teenage boys. And hisasthma.
Also, the Militia soldier mentioned above. He really wants revenge on Danny.
Armadillo Quintero silences a prepubescent witness against him by raping her and tattooing a dove on her cheek to "claim" her.
Antwon Mitchell murders a teenaged informant, both to pay her back for snitching on his drug operation and to frame two cops for her death.
Sons of Anarchy: Cameron Hayes holds Abel Teller at knifepoint and then kidnaps him. As part of her effort to get him back, Gemma Teller holds another baby at gunpoint.
Starsky & Hutch: In the episode "The Crying Child", a teacher discovers that one of her young students has deep gashes on his back, and the titular duo starts to look for who had harmed the kid and manage to find out it was the boy's own mother.
Geriatric antagonist Gerard Argent is a master of this trope with teenagers: he violently drowns the (admittedly not innocent) Matt , kidnaps Erica and Boyd and holds them in prolonged electrical torture, kidnaps and beats the absolute snot out of Stiles, stabs Scott in the gut, and tries to murder his own granddaughter, Allison. It's a little disturbing to watch.
In the beginning of Season 2, Isaac Lahey's father throws a glass at him and the shards nearly put his eye out. We also see implied evidence of his severe physical abuse.
Walker, Texas Ranger: More than once, the villains would hit children — sometimes beating them viciously or otherwise putting them in extreme danger — as they would hold them hostage, either as bargaining tools or just to show how ruthless and sadistic they were, always without conscience or fear of the consequences. Examples include burying a busload of children inside a school bus at a landfill, and burying another alive in a casket. There were other episodes involving child abuse, but said abuse is by a parent who happens to be the main villain and is used to frame his evil personality. Walker and the Rangers would arrive to give the baddies a taste of their own medicine, with extra force as Walker had zero tolerance for child abuse.
The Wild Wild West: Many of the villains. Highlighted in "The Night of the Amnesiac" when Furman Crotty steals and then holds for ransom the vaccine supply during a smallpox epidemic. While trying to recover the vaccine, one of the main characters comes across a funeral service in progress. The coffin is a small one. Very, very small.