Do you know that often repeated expression out of the mouths of babes? This phrase comes from the Gospel of Matthew, and Rudyard Kipling, among others, used it.
What does this expression mean? It refers to how children will speak with honesty and innocence. Sometimes they may be too honest to know better than to keep some things secret, and too innocent to grasp the meanings of things they’ve said. As you read here, adults wish these children didn’t open their mouths at all.