"Poster child" is an idiomatic expression that refers to a person or thing that is seen as emblematic of a particular group, cause or idea. However, there are several synonyms that can be used to replace this term. One alternative is "symbol," which suggests a visual or nonverbal representation of a concept or group. Another option is "exemplar," which implies a model or prototype of excellence. "Mascot" is also a possible synonym for "poster child" as it suggests a symbol of good luck or identity. Lastly, "emblem," which refers to a design or figure that represents a particular concept, can also be used instead.