[Size30 plot based of the manga the promised Neverland]
the year 2045, there exist special orphanages that raise children with the utmost care. They are well dressed, well fed, and well educated by the women that run these orphanages, called 'Mothers' or 'Mamas', respectively. They are raised with care for up to 12 years before they are adopted out, though some are adopted at roughly six years of age. Adoptions are bittersweet, but supposedly are cause for celebration. Surely despite the children who've left never writing back to the orphanage, they must be happy?
Children do not leave orphanage grounds and are prohibited from overstepping known fences, walls, or boundaries, and most heed these warnings, as it is to their understanding that monsters await them on the other side. Mothers keep a trained eye on all their children using radars within pocketwatches, as every child has a microchip put in their right ear at birth as well as 5 digit tattoo codes on their necks or chests with which to identify them.
Most children age and mature without ever questioning it, but there are a select few, the most gifted kids, who begin to see patterns and inconsistencies alike. As Mothers proctor daily tests to all the children throughout the span of their lives in an attempt to enrich their growing brains. Their intelligence grows and of course eats away at their prior bliss.
Where are the 'adopted' children really going?
Why have no children seen any/many other adults, and why can't anyone but their Mother leave the grounds? What lies over the fences and the walls?
A WORLD OF DEMONS
All orphanages are to be considered farms, for lack of a better word. Children wear the same crisp clothing and are kept free of injury, bred specifically to become food for a very large, amorphous race of beasts, or 'demons', that must consume other species' brains and bodies to survive. They 'are' what they eat. Orphanage caretakers/human Mothers work directly for these creatures as farmers both in order to survive, themselves, and to provide nutritious human brains to the demon race. The more intelligent the meal, the more intelligent the beast. As such, eating humans on a regular basis is the only way they can maintain their bodies, their society; Their sophistication as a 'people'. The more wealthy, more prominent demons have the luxury of eating what we might call prodigies, the most intelligent livestock. At between 12-18 years of age, when the brain has enlarged, they are ripe for the picking.
These children are from the special orphanages mentioned before, and are not harvested before the age of 6. Those harvested at age 6 have likely not progressed well in their tests, and so are still intelligent but are not quite prized livestock. Those that progress substantially and impress their Mother remain until they turn 12-18 (depending on the orphanage), and are promptly... 'adopted out', to a wealthy diner.
Demons come in many shapes and sizes, but the only ones capable of speech are the ones that eat humans. For middle-class or even impoverished demons, factory farms exist to feed them. Human children residing in factory farms are force-fed through tubes, have never known a language, and are so bloated and inept that disconnecting them from their feeding tubes would kill them. They are cheap and are dismembered down to their fingers and toes and are sold at demon street markets in jars and in slabs alike.
Rarer but by far a greater threat to demon society are escapees that hail from Lambda facilities. These are demon-run establishments that specialize in experimentation, and it isn't uncommon for certain abilities to materialize as a result of the excruciating experimentation process many of the children there are forced to undergo. Greatly enhanced speed, strength, intelligence, or even otherwise superhuman traits may awaken as a result. When Lambda children escape, they are a major threat to the demon race.
So what would the world be like for those that recognized their fate for what it was? What options are open to those intelligent children whose will to live replaces their ignorance?
ESCAPING AND SURVIVAL
Beyond the walls, the world of demons is a rugged one. Dense forests, flesh-eating flora and fauna, and even feral demon beasts roam freely. Better still, those that control the orphanages will be sure to hunt the children down themselves, be it by their own hand or by use of demon hounds to aid their search.
Hiding to survive won't save them everytime... They'll have to learn to kill demons, as well.
Other rebel humans will have left bunkers, clues, supplies, and various other things behind in an attempt to help runaways survive and adapt. After escaping and finding a way to remove their tracking chips, they must fight to live. Killing demons is the only way to ensure future generations of children will no longer be raised as cattle.