I'm sure I wasn't the only one who thought from the title, it was a manga where the girl gets summoned to become a Nanny for the for Demon Lords kids, whose mother had been killed by the hero, was definitely not expecting her to Actually be the mother. Though looking at the front page art from the first chapter, the clues were there they were her actual children.
That said I expect if this how children are conceived it would explain why the Demon lord is such a grumpy man.