Having behavior or thinking that is immature or childlike; appearing childish.
彼の言い訳は子供っぽくて説得力がない。
His excuse is childish and unconvincing.
そのデザインはかわいいけど少し子どもっぽいね。
That design is cute but a bit childlike.
もう少し大人になって、子供っぽい振る舞いはやめてほしい。
I wish you'd grow up a bit and stop acting so childish.