immature, naive, or puerile in behavior or thinking

puerile or inexperienced in skill, thought, or behavior

childish nature, infantile air, or juvenile quality in manner or appearance

kindergarten (Taiwan usage)

still possessing the innocence of childhood

a young child or youngster

naive, immature, or childlike in manner or artistic style

immature and feeble

(entomology) naiad

little girl (of toddler age)

sweet and innocent