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.
Battlestar Galactica (2003): In the pilot miniseries, shortly before the Cylon attack, Six snaps a baby's neck while the mother is distracted. There has been much fan debate on why she did that.
Barbarians Rising: Boudica's segment is one of the nastiest. Her segment starts with has her flogged and her teenage daughters raped by Roman soldiers. In retaliation, Boudica and the Britons burn three Roman cities to the ground and slaughter the inhabitants to the last man, woman, and child, including personally setting fire to the Temple of Claudius with women and children inside begging for mercy. The segment ends with Boudica's battlefield defeat; one of her daughters is run down by a Roman cavalryman.
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."
In season 2, some street drug dealers draw 11 year old Tomas Cantillo into their gang, having him kill Jesse's friend Combo as a rite of initiation. He's later killed by said dealers, either on their own accord or on the orders of Gus Fring, upon being told that they can't use child assassins.
A more significant example comes when Brock Cantillo 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 Alquist shoots 10 year old Drew Sharp for accidentally stumbling 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.
Todd's uncle Jack Welker and his gang of Aryans get Skyler's and Jesse's cooperation by threatening to kill, respectively, Holly and Brock. Considering all the other horrible things they had done (as these are the same guys who organized the shankings of 10 inmates in 2 minutes, who massacred Declan and his crew, who killed Hank and Gomez, and who kill Andrea when Jesse makes an attempt to escape their compound), it's almost certain they would go through with their threats.
Monroe and his group have no qualms attacking Bayarmaa even though she is carrying an infant.
Natalie Rice of Burn Notice held children hostage so their father would act as her fall guy. When held at gunpoint by Fiona, she blended into a passing crowd of children so she would back off. As extra incentive Natalie warned she would kill one at the first sign of trouble.
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.
Dead Like Me: The series finale has a serial killer who targets several people on Halloween. Two of his victims are a little boy and a teenage girl. George herself was almost a victim of the killer when she was a child.
The reapers also count to a certain degree, as their reaps will occasionally be children. The reapers are usually clearly upset at doing so and only do it because they have to.
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).
Jaime Lannister defenestrates note Pushes out of a window.Bran Stark, rendering him temporarily comatose and permanently crippled after the boy witnesses the twincest. He "hoped the fall would kill him".
In fact, this series has quite a bit of child deaths at the hands of villains and anti-heroes, such as The Hound cutting down the butcher's son, Lommy being speared through the throat by Polliver, and, most gruesomely, corrupt City Watch commander Janos Slynt killing a newborn baby in front of its' mother, which is part of a citywide massacre of bastard children of King Robert; luckily, Janos gets his comeuppance in various ways.
Quite possibly the most innocent character on the show, Shireen Baratheon, is burned alive by her own parents and Melisandre as a sacrifice.
Ramsay feeds his newborn half brother (and step mother) to his hounds, then follows that up by shooting Rickon Stark in the back during a prisoner exchange
Brandon Stark nearly killed Littlefinger, slashing him about a dozen times and would have killed him if Catelyn hadn't intervened.
After Jon's resurrection, he must have one of his murderers, Olly, hanged for treason along with the rest of the men who killed him. This weighs heavily on him, though, and itís clear how much this devastates him, leading to how utterly destroyed he feels after this and everything.
Driven mad by grief and rage, Karstark brutally and mercilessly murders Martyn and Willem Lannister; two young children who were barely squires, all for the crime of having a Lannister name.
Tywin Lannister is infamous for this, as discussed by Lady Stark who was dead worried about Sansa and Arya because the Targaryen children were butchered in their sleep on the orders of Tywin. The children of House Reyne weren't spared either, as Cersei boasts to Margaery.
During the siege of Riverrun, Jaime threatens to kill Edmure Tully's son if Edmure doesn't get Riverrun to surrender.
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.
Francis Dolarhyde on Hannibal kills entire families, and seems to save the children for last.
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.
In one episode of Horatio Hornblower, Horatio only just manages to talk an expatriate French count out of shooting a young boy for singing "Le Marsailles" when the count returns with the anti-Napoleon rebels to take back his land.
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!"
Justified: 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.)
Legend of the Seeker: Darken Rahl, unsurprisingly. Not only ordering every baby boy in a village massacred, but he also promises to have Princess Violet be made a slave as punishment for her mother's failure. He ordered his own son with Cara killed at birth as a potential threat (in the books he wanted a gifted child as his heir, and killed his children because they were ungifted).
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.
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.
In another episode Sam was steeling himself to kill a little girl possessed by Lilith. At the last minute Dean, temporarily given demon sight because of his impending damnation, tells him that Lilith isn't in the girl anymore.
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.
Torchwood: In the third season, Children of Earth, the 456 remotely mind control all of Earth's children to coerce the world's governments into letting them kidnap several million kids in order to make drugs out of them. In the final episode of Children of Earth, Jack Harkness uses his grandson Stephen to transmit a lethal signal to drive away the 456, killing Stephen in the process.
Numerous antagonists in The Walking Dead have demonstrated that they have no qualms with hurting or killing children. Notably, one man tries to rape Carl in the season 4 finale while another threatens to break baby Judith's neck in the season 5 premiere.
A surprising non-villain example happened in the season 6 midseason premiere when Michonne kills Ron, after he almost shoots Rick, blaming him for his family dying. But as he dies he gets one shot off that hits Carl in the eye.
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.