"The 'red herring' which swallowed boy number four is not an alteration of the [original] text [of the poem], though Christie must have found the appearance of the phrase amusing" (Sanders and Lovallo, The Agatha Christie Companion, 179). In mystery stories, a red herring is a clue introduced to purposely distract the reader from the actual murderer.
Why "red herring"? The term refers back to the "smoked herring . . . supposedly used by fugitives to put bloodhounds off the scent (1680s), hence metaphoric sense (1884) of 'something used to divert attention from the basic issue'" (Online Etymology Dictionary).