Synonyms for Childish:

adj.

all (adjective)
infantile.
babyish (adjective)
infantile.
immature, silly (adjective)
foolish, adolescent, babyish, puerile, infantile, green, young, callow, youthful, naive, baby, juvenile.
juvenile (adjective)
infantile, adolescent, young, kiddish, juvenile, immature, youthful.
young (adjective)
puerile, junior, immature, green, adolescent, boyish, blooming, child-like, callow, inexperienced, fresh, teenage, young, girlish, juvenile, unripe, youthful, pubescent.

n.

childhood, boyhood, girlhood, silly, development, foolish, weak, developmental, adolescence, absurd, early. ludicrous, old, over-the-top, senile, not in your right mind/not right in the head, laughable, ridiculous, mad, crazy, insane. youth. childish (noun)
girlish, infantile, irresponsible, puerile, silly, foolish, juvenile, immature, babyish, boyish.

Other synonyms:

girlhood, developmental, boyhood, development, irresponsible, over-the-top, perverse, adolescence, ridiculous, unreasonable, absurd, youth, Sophomoric, childhood, insane, irrational, wrong, ludicrous, laughable, crazy, mad. early, extreme, babyish, silly, hysterical. infantile
baby.
ingenuous
naive.

Usage examples for childish

  1. They were very neat and pretty, and Ruth, who loved all beautiful things, smiled on them through her tears, and reaching out her hand, felt of them with simple, childish delight. – Stories of Many Lands by Grace Greenwood
  2. I was feeling very foolish and childish – A Little Union Scout by Joel Chandler Harris
  3. Has the learned Cassiodorus grown childish – The Scarlet Banner by Felix Dahn