Eh, I don't blame her for being naive, this seems like a very ruthless household, and based on my measly 6-chapter-based knowledge of the series, I think she's just wary of the Duke because she's literally seen him with a freshly killed (or about to be freshly killed), mutilated corpse with blood dragging everywhere. So a pretty negative first impression.
Whereas it seems like the novel this world is based on failed to mention Ain's yandere-esque personality and tendencies (alongside other details like his allergies).
So basically her knowledge consists of a novel that is missing a lot of detail and 2 very different first impressions regarding the father and son. Factor in that she's a daycare teacher, and it makes sense why she's very biased towards the cute, baby-faced, smiling boy compared to his towering, intimidating, often-bloodstained and cold father that she doesn't really know. It's not surprising that she's very uncomfortable around the Duke and wants to spend pretty much all her time with Ain.