It's a joke, and was readily appreciated as such by both my kids. There's a sequence of things to which you are saying "goodnight", and to include "nobody" and "air" in that list is unexpected, funny and thought provoking to a small child.

"The nothing itself nothings" is Heidegger, and while I agree metaphysics often amounts to thinking far more deeply about meaningless quirks of language than they deserve I don't see the relevance to a book for preschool children. Do you see all absurdist linguistic humour as "imprecise logocentric thinking"?