“Children are not a distraction from more important work. They are the most important work.” Attributed to C.S. Lewis
It doesn’t sound like him. None of the hits on the Internet return a primary source. It’s not on Wikiquote. It doesn’t come up with anything on Google books.
I’m calling it fake. I think it gets attributed to him because of the Narnia books.
Update for 8/23/2015
Since I first published this post, I’ve gotten several comments on it. One set of comments is associated with Dr. John Trainer, who claims to be the originator of the quotation.
Another set of comments claims that it is CSL, “from a speech he made in 1952 at the Library Association, titled ‘On Three Ways of Writing for Children.’ The speech was later adapted into an essay in Lewis’ ‘Of Other Worlds: Essays and Stories.’”
I don’t own that book, but I have a bibliophile friend who does, and he says that the quotation is not in the essay in the book. I couldn’t find it searching the text of the book on Google Books either. The essay also appears in Lewis’s On Stories, but I can’t find the quotation there either. I don’t have access to the speech so I can’t check there.
Absent a specific page number of a specific edition of a specific book, I remain skeptical that Lewis actually wrote this and will give the nod to Dr. Trainer unless someone can come up with a solid earlier citation.