Someone who is ingenuous shows a childlike innocence, trust, and openness. One of the things kindergarten teachers value is the chance to work with kids while they're still relatively ingenuous––their open, trusting natures are a joy.
A near synonym is naive. Its opposite is disingenuous, which means "giving a false impression of being honest and sincere." Don't confuse the word ingenuous with the similarly spelled ingenious, which means "very smart or clever." Ingenuous is from Latin ingenuus "having the qualities of people born free, noble, honest, open," ultimately formed from the prefix in- "in" plus gignere "to produce."