I'm confused because the summary clearly states who the child is. The chapters also confirm it. He's the child of the former Baltgar Marquis. Who was killed by her stepbrother, the son of the man (Berend) who killed the FL's father (the Count). He killed whoever tried to become Zephion's guardian. Since the child's parents are dead, it's likely the newest guardians whom he just killed and he called the FL over to the mansion after he finished. It's unlikely that Kaelid has lived in the Baltgar mansion for a very long time, if at all, since it looks like he's unaware that the boy is still alive and hiding in the mansion. So the reason Kaelid claimed that he was going to make an heir with her was most likely because of either the bed they were doing it in or him at least inheriting the title.

wait I’m confused?? so Kaelia is her “stepbrother” but is the kid his son? but the marquis is dead ? maybe it’s his little brother? idek man