I believe your question refers to Matthew 18:6, which is part of this particular teaching (Matthew 18:1-6): "At that time the disciples came to Jesus, saying, ’Who is the greater in the kingdom of heaven?’ And calling to him a child, he put him in the midst of them, and said, ’Truly I say to you, unless you turn and become like children, you will never enter the kingdom of heaven. Whoever humbles himself like this child, he is the greatest in the kingdom of heaven. Whoever receives one such child in my name receives me; but whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened around his neck and to be drowned in the depth of the sea" (RSV).
We are born fully aware of our innate Christ identity and our Oneness with our spiritual Source. As we move deeper into our human experiences, we begin to lose contact with that Christ identity, and to forget our Oneness with Spirit. That’s why Jesus always taught that children are to be honored and appreciated for the spiritual Light and Love that they have not yet lost sight of. We must ourselves "become as children"—return to that earlier spiritual awareness and expression that we had before human concerns began to distract us. If, instead of receiving and appreciating the Christ awareness that children offer, we allow ourselves to "infect" children with our own fears and negative energies, we are making a serious and damaging error that will weigh heavily—like a "millstone"—on our own consciousness. The challenge of lifting ourselves out of error consciousness to the sense of Oneness we once knew is great. It becomes infinitely greater if we allow our fears and false beliefs to lead others into error thought as well.