When I was really young everything went smoothly.
The kanji means infancy. The adjective version refers more to your entire child-times, including baby times. But, if you're described as being like a child from child times, what are you called? Childish.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: When you were childish and immature you were sent off to do prayers to ask for forgiveness. One of the things you had to do was sing "hosanna (おさな) in the highest."