The phrase "most childing" can be substituted with a few different synonyms that convey similar meanings. One such synonym is "most childish," which implies immaturity or an immature behavior. Another synonym could be "most juvenile," which is similar in meaning to "most childish" but can also suggest youthful enthusiasm or energy. "Most puerile" could also be used, which specifically connotes a lack of adult sophistication or refinement. Ultimately, the choice of synonym will depend on the context in which the word is being used and the desired connotative and denotative meanings.