The best lore reason I can imagine is, you don’t see feral children because they don’t want you to see them.
Human beings are animals, and like all animals are subject to the same natural forces as every other animal. Human childbirth and parental care appeared long before doctors, nurses, or midwives. Human childbirth happens naturally, whether there’s a helper there or not, even in this day and age. Without helpers you’ll see increased maternal and infant mortaility in childbirth, especially at first because natural selection against very large infants has been severely relaxed for a long time. But childbirth mortality will not rise to the level of eliminating the species, which is what “actually no children” means.
Human infants require a lot of parental care to survive. Not a big deal. So do kangaroos, and they’ve survived a very long time. The need for parental care is not a reasonable reason for human extinction. Humans understand not caring for children means extinction. Survivors will care for and protect their children, because survival means reproduction, plain and simple.
Ferals will be as good or better protectors of their children than non-ferals, because they are the true survivors, doing whatever it takes. Wiping out adult ferals wipes out their children. It’s an essential truth that makes auto-attacking-to-the-death ferals unrealistic without lore that they have no other ferals back at a protected home caring for young, and are suicidal. Ferals do what comes naturally, it’s kind of their definition. Sex works the way it does in humans for a reason, that reason is to ensure humans reproduce, and it works extremely well. In the time it takes people to become feral, they will also make new families.
Realism for ferals would be relatively safe home sites, almost certainly run by the females (males MAY have “leadership” roles in home, enforced physically, but the females make sure the home site does what it must do, maximize childhood survival). Males scavenge, hunt, chase off threats including unknown humans, but run away when they are hurt, because their death weakens the entire clan. If home sites are all off camera, because reasons, then the ferals you see will all be male, will run away when badly hurt, will usually break off their attacks when you run away from them, and will heal because they have a safe haven to fall back to. Their primary focus, regarding you, is keeping you away from their family. The “reason” you don’t see feral children, is because they don’t want you to see them.