Kaylee looks -- almost startled, for a moment; she picks up her teacup and doesn't respond immediately.

When she does, it's with a slightly wry half-smile. "That's...not exactly the only thing I'm worried about, but it's a pretty gorram big one." And then the wryness is gone. "Thank you. I appreciate that."

That's not the same as saying that she believes him; she's smart enough to know that that's a big promise, that nobody will get hurt.

She's also smart enough to know why he's making it.