"When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things." (1 Corinthians 13:11)
Jejune
So, there's that world of difference between the above referenced childishness and the "childlikeness" spoken of by Jesus. The Bible may be decried all the livelong day for inconsistencies and paradoxes and contradictions. But this isn't one of them for God's sake. Granted, Paul is speaking through the King James translation but "childishness" is "childishness". No way around it except by growing up. If you're reading this and you've done that (grown up), then you know what I'm talking about. Your tantrums (call it "emotional outburst") and babytalk ("vocalese") are of no effect here. But if you feel so inclined, know that God still sees you and loves you as His baby and understands the wordless depths of your being being represented by blunt feeling and simple color.
"Then said I, Ah, Lord God! Behold, I cannot speak: for I am a child." (Jeremiah 1:6)
Puerile
This word sounds so childish, to me. I suppose because that's what it means. Interesting thing, were you to elide the first "e" and interpose it between the "j" and "u" of the first bolded word up there, you'd get "jejeune". Which isn't a real word, but harks back to the French for "game" and "play" (jeu). Two childlike, if childish things you shouldn't "put away" even as you become a man, or woman.
"And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the Kingdom of Heaven. Whosoever therefore shall humble himself as this little child, the same is greatest in the Kingdom of Heaven." (Matthew 18:3-4)
And "purile" is not a real word either. But it reminds me of the heart of a child (purée—baby food). Pure. This is what God is aiming at and it's what can be regained, if retained, by knowing Jesus. The key to returning to that state is humility. Humility is a voluntary act. One we can choose with each moment we encounter. To choose to bring God in to our circumstances and allow Him to diffuse the childlike atmosphere we waded around in at a young age but perhaps (most likely) lost over the years. The beautiful thing is, now, as God regifts it to us moment by moment, He'll also show you how to maintain it in light of a world that would put a damper (as opposed to a diaper) on the childlikeness of God.
"But Jesus said, Suffer little children, and forbid them not, to come unto me: for of such is the Kingdom of Heaven." (Matthew 19:14)