babyish

lapsellinen, vauvamainen Like a baby; childish; puerile; simple.
1953, (w), (w), Collins, 1998, Chapter 9,

As soon as the King and the rest of the hunting party had set off, she began making a tour of the whole castle and asking questions, but all in such an innocent, babyish way that no one could suspect her of any secret design.

For a young man, he has the most babyish voice Ive heard.''