Matthew 18:1-4 says:
1At that time the followers came to Jesus and asked, "Who is the greatest in the kingdom of heaven?" 2Jesus called a little child to come to him. Jesus stood the child before the followers. 3Then Jesus said, "I tell you the truth. You must change and become like little children [in your hearts]. If you don't do this, you will never enter the kingdom of heaven.4The greatest (most important) person in the kingdom of heaven is the person that makes himself humble like this child. (Easy-to-Read Version)
King James Version: 1At the same time came the disciples unto Jesus, saying, Who is the greatest in the kingdom of heaven? 2And Jesus called a little child unto him, and set him in the midst of them, 3And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven. 4Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven.