WordNet sense 1 (exhibiting childlike simplicity and credulity):

simple, wide-eyed, dewy-eyed, round-eyed

WordNet sense 2 (marked by or showing unaffected simplicity and lack of guile or worldly experience):

naif, naive

WordNet sense 3 (befitting a young child):

childly

WordNet sense 4 ((used of living things especially persons) in an early period of life or development or growth):

immature