She is too young to not take this seriously, younger than she looks, and the situation doesn't really make it sound like a joke to her, either. Plus she hasn't encountered the concept of monsters living under your bed yet, partly because she's never had one herself.

Her forehead knits as she tries to put a description to his words.

"...A bird under a bed would not be scary but helpless and very easy to catch." Or she can at least not think of any other being that would have a beak.