Blame the manga for cuting the other students chapters.
We're supposed to already know Sophia (the baby) because she's is a recurring character in Shun (the other Main Character of the novel) chapters.
Sophia
The point is that Shun' story is happening 16+ years in the future. A lot of things that happens in Kumoko' story are foreshadowed first on Shun' story.
- The Demon lord appears on volume 3 in Kumoko' story meanwhile she's mentioned in Shun' story since volume 1
- A lot of side effects of several skills are mentioned in Shun' story because he has a Skill Encyclopedia
But this conection works in both ways:
- While reading the skill encyclopedia Shun mentions that there's not a skill that boos EXP, obviously we know that that's a lie because Pride. This means that someone is trying to hide the existance of the Ruler Skills.