Definition
This idiom is used when a child says something surprisingly wise or insightful.
When I was complaining about my job, my four-year-old son said, 'Maybe you need a new one!' Out of the mouths of babes.
My niece said that we should always be kind to each other, out of the mouths of babes.
In a discussion about the environment, a child said, 'We should stop polluting the earth.' Out of the mouths of babes.