He did say, after all, we needed to be childlike, not childish. Nor did Jesus intend us to understand from this that we must strive to develop some of the qualities of a child, such as innocence or purity or a simple, childlike faith. That's often the way it's interpreted, but I think it's wrong. After all, children can be viewed in many ways. Sometimes they are sweet and loving and trusting, and willing to take things on faith if they can't understand it, but they can also be stubborn, self-centered, willful and cruel.
Children are no more free from sin than adults are.
Jesus isn't holding children up as models of innocence. His point wasn't that the disciples should try to develop certain child-like characteristics. I think his point was that we all have to be willing to become what a child, in itself, was, at least what it was in Jesus' culture. The key thing was the position of the child, not its subjective qualities. In the ancient world children were non-entities.