childish; childlike; immature; infantile; puerile
その反応は大人にしては子供っぽい。
That reaction is childish for an adult.
昔は子供っぽかったが今は落ち着いた。
He used to be childish but has settled down now.
彼が子供っぽく駄々をこねたので周りは対応に困惑した。
His childish tantrum left those around him at a loss for how to respond.