There is a simplicity and gentleness about little children. The disciples thought Jesus was too busy to be bothered by children. But this was not true. Jesus understood that Jewish mothers wanted Him to give their little ones a blessing. Therefore, when the disciples would spare Him this supposed annoyance, He rebuked them. Then Jesus called the little ones and blessed them.
The statement, "For of such is the Kingdom of Heaven," does not mean that the Lord is electing or selecting little children for His joint-heirs in the Kingdom. We must interpret Scriptures with other Scriptures. Remember Jesus's words that only "overcomers" can rule with Him in the Kingdom (Revelation 2:7,11,17,26; Revelation 3:5,12,21). Again, Jesus said that whoever would be His disciple must take up his cross and follow Him. Such things could not be true of infants.
Heart purity and simplicity are necessary for the Kingdom, but more than these are required. The text of the question should be understood in harmony with our Master's words, "Except ye . . . become as little children ye shall not enter the Kingdom of heaven" (Matthew 18:3). The emphasis lies upon the word "as" in the sense of like. Only the child-like and trustful disciples can experience God's highest favor–a share in His Kingdom.