The 456: really Extra Sufficiently Advanced Aliens, or just bluffing? They clearly do have advanced virology, but we never see any other technology, and the only physical defense they rely on was mundane bulletproof glass. Making all the children of Earth speak in unison isn't much more impressive than what the Sycorax did, and they were bluffing. This ambiguity only makes the story even more tragic, but left some viewers wondering if UNIT or the British government had really tried everything before surrendering.
Or even worse, what if the 456 Ambassador was the only individual responsible for the entire incident? A junkie who'd conned the entire Human race into supplying him with children under threat of the rest of his species, who might have no idea what he was up to?! Much like the Slitheen are to the Raxacoricofallapatorians, this could be simply one bad egg in an otherwise peaceful species?!
And You Thought It Would Fail: Most of the cast and crew felt that reducing the series to five episodes and moving it to what was widely considered a graveyard slot would result in it not being a major success. As it turns out, Children of Earth garnered very respectable ratings, is widely regarded as the best out of the four Torchwood seasons, and is considered by some to be one of Russell T Davies's finest works.
Broken Base: Ianto's death (and/or the way it was handled) and whether Jack's actions were justifiable in context.
The Ambassador is a member of a race of Starfish Aliens called the 456 who incorporate prepubescent children into their physiology, as their bodies produce hormones that act as euphoric drugs to the 456. The children are kept as perpetually-childlike human reefers, one child shown as having been rendered hairless and immobile as a result of forty years of being used as a drug by the 456. After initial negotiations under falsely benevolent pretenses in 1965, the Ambassador returns in 2009 and bargains with the British government to take ten percent of the world's children under the threat of wiping out all humanity; to prove its power, the Ambassador unleashes a lethal virus throughout the Thames House that leads to the death of almost everyone inside, including Ianto Jones. Although only one member of the 456 is ever seen, the Ambassador shows itself to be exempt from Blue and Orange Morality, and expresses callous disregard for the living beings it is harvesting. The Ambassador is reflective of Torchwood'sdarker nature and is one of the most disgustingly evil villains to ever come out of the Whoniverse.
Prime Minister Brian Green calmly allows 10% of the world's children to be sold as narcotics. He orders Frobisher, the man who's been most loyal to him, to give up his own children just to make the cover story he's created realistic. This leads him to shoot his family and himself to spare them the horror. After all the horror and pain, Green's first thought is how he can blame the Americans.
Die for Our Ship: Gwen is perceived as getting in the way of Jack/Ianto, especially after Ianto's death.
Growing the Beard: Very contentious, but this miniseries was the first real critical and ratings success for the show.
Hilarious in Hindsight: Gwen's "Where is the Doctor" angst at the top of Day Five, in light of Peter Capaldi's casting as the Twelfth Doctor. But it's also...
Harsher in Hindsight: Later on Day Five, there's Bridget Spears' monologue about John Frobisher, in which she says that no matter how history chooses to remember him, he "was a good man." Earlier in Gwen's monologue, she theorizes that the reason the Doctor doesn't save humanity from every disaster that comes upon it is because he's seen how evil humans can be; "Sometimes the Doctor must look at this planet and turn away in shame." Eventually, there will be a markedly Anti Heroic Doctor who wonders if he is a good man or not in his efforts to help others, especially humans...who has the face of an Anti-Villain, a good man, who could not bear the shame and evil of one of Earth's darkest hours.
Prime Minister Green, when he orders Frobisher, his most loyal ally, to sacrifice his own children to the 456.
"I'm sorry John. I'm really very sorry...and I'm really very busy."
The ministers of the government choosing to protect their own children no matter what and then taken even further when they all agree to sacrifice the poor and disadvantaged children to save the respectable middle class ones. Though they still don't end being as unsympathetic as Green, since they at least admit to their bias and aren't exactly happy about their actions.
Playing Against Type: At the time this was made, Peter Capaldi was best known for playing Magnificent BastardAnti-Hero Malcolm Tucker in The Thick of It — a man with no scruples or shame over the corrupt government he works for. In Children Of Earth he plays John Frobisher, a Butt MonkeyAnti-Villain who is so ashamed of his government's acts he commits suicide after murdering his family to spare his family from being turned over to the 456. The two characters are polar opposites.
Retroactive Recognition: A bit of a doozy, as Anti-Villain John Frobisher is played by none other than Peter Capaldi, later to be cast as the Twelfth Doctor! Word Of God (both Russell T. Davies and Steven Moffat) eventually provided an explanation for this and Capaldi's other role in the Whoniverse, Caecilius in the Tenth Doctor story "The Fires of Pompeii", via Retcon. Twelve's face resembles Caecilius's because — as Caecilus and his family only survived the destruction of Pompeii because the Tenth Doctor saved them — he subconsciously chose it as a reminder to always save whomever he can, however he can; this is revealed in-universe in a key scene in "The Girl Who Died". All There in the Manual material continues the story: Frobisher was a descendant of Caecilius, and his Pater Familicide was Time "compensating" for the Doctor's meddling. But that just strengthens the Doctor's determination to not let the fates hold sway over precious lives, giving his choice of face further meaning.
What an Idiot: Despite being a professional alien fighter by trade, Jack Harkness simply swaggers into Thames House and announces "no" to the 456 like a giant swaggering cock, then acts surprised when they kill everyone, including Ianto. One would think he would, I don't know, use science to figure out a weakness beforehand (which is kind of his qualification for the job)?