1580s, "proper to a child," from child + like (adj.). Meaning "like a child" in a good sense (distinguished from childish) is from 1738. "Sir Gawain and the Green Knight" (c. 1380) has child-gered "boyish, lighthearted."
Childlike and childish express that which is characteristic of a child, the former applying to that which is worthy of approbation, or at least does not merit disapproval, and the latter usually to that which is not: as, a childlike freedom from guile; a childish petulance. To express that which belongs to the period of childhood, without qualifying it as good or bad, child or childhood is often used in composition .... [Century Dictionary, 1897]