noun-modifying (irregular)
Describes behavior, ideas, or appearance that is immature or childlike in a negative or critical way. Often used for adults acting like children.
彼の子供じみた行動には呆れてしまった。
I was disgusted by his childish behavior.
そんな子供じみた言い訳は通用しないよ。
Such a childish excuse won't work.
幼稚 is a more formal or written term for 'childish' or 'immature', often used in compound words like 幼稚園 (kindergarten). 子供じみた is more colloquial and directly describes something as resembling a child's behavior.
Compound of 子供 (child) + じみた, the past tense of 染みる (to soak into, to be tinged with), meaning 'tinged with childishness'.