Final answer:
A child saying 'Juice!' to express a desire for some juice is using telegraphic speech, which is characterized by conveying the key message with essential words, mirroring how a telegram would be composed.
Step-by-step explanation:
A child who says "Juice!" to mean, "I want some juice!" is using telegraphic speech. This term is used to describe early speech stages where children communicate essential words and omit less critical words, similar to a telegram's concise language. It is not considered overextension, which is applying a term too broadly, nor underextension, where a term is too narrowly applied. Additionally, it is not babbling, a stage involving repetitive sounds without meaning.
Throughout language development, children utilize different strategies and stages to convey meaning. From the one-word stage, they progress to combining words and building their lexicons, often simplifying complex sentences to just the key words that convey the essential message, which is precisely what telegraphic speech is about. Although a child might still be learning the intricacies of language and grammar, the ability to communicate complex ideas through minimal words is a critical step in language acquisition.