She points to herself, then points to the cabinet. "In," she says. By 'in' she does not mean, "putting babies in cabinets is the stylish thing to do right now", though it is very possible that Esmé Squalor would think so. However, this typist does not have any evidence to point either way for that. It is just as likely that putting babies in cabinets is currently out. What Sunny means by 'in' is that she wants to see if she can fit inside the cabinet. It's a science experiment.