Use 'ingenuous' to describe someone's pure and simple way of thinking, like a child's. His ingenuous question about where the sun goes at night made everyone smile.
Pro Tip 2/3
Not Cynical
'Ingenuous' suggests a person is not sly or skeptical, often refreshing in complex situations. Her ingenuous trust in people was rare in the cutthroat business world.
Pro Tip 3/3
Unintentionally Honest
Sometimes 'ingenuous' means someone is honest without realizing the full context. He gave an ingenuous opinion about the painting, not knowing it was by a famous artist.