I guess he that's checkmate for her. He basically called her out to her face that he knows she's feigning innocence. Looking forward to next chapter where we see her reaction.
@itsnottme His brain should be almost fully developed by this time. What's more, child's personality is not formed only by brain development, what shapes it are experiences. Ars was born with his past life experience, so it must have a leading role in his character development, childlike experiences were already pushed aside and affected by his memories. So, by all means, he should resemble an adult, not a fidgety boy(at least if he wasn't like that in his last life).