For me, fluff, like beauty, is in the eye of the beholder. A story may have deep meaning for the author, but little or no meaning for a discriminating editor. All fiction writers know that a quality and engaging story must be developed with a a plot and a character the reader will either love or hate. It must also have an arc that leads the reader to the crescendo or climax.
But there are times when the only goal to a story is to make a child laugh.
Think "Knock, Knock" jokes. There isn't a plot, or an arc, or most of them don't even make sense, but kids love telling them and can't help but laugh when they do. (Knock, knock. Who's there? Stinky. Stinky who? Stinky, dinky LaRue, and that's you!)
Or, how about all the books about farts, burps, and other bodily functions? Adults may think books like these are trivial, superficial, or even fluff, but kids love them.
Non-fiction, however, seems to be different, and if you don't believe me just take a look at a daily newspaper or one of the celebrity magazines. But back to fiction . . . .
So in conclusion, writing for kids can be serious, imaginative, mysterious, engaging, humorous, thoughtful, soul-searching, adventurous, fantastical, and even cotton candy FLUFF if it appeals to even one child. After all, isn't that why we write for children?