I think you're neglecting the most obvious reason people use words: to communicate something. "Youngling", which is not a new term really, is used in Star Wars media to refer to juveniles of a variety of species. Why not just use the word "child"? Well, because a child is a juvenile human being, with the timeframe and connotations that come with that. Using "youngling" makes the biological diversity immediately apparent to readers, and has the in-story effect of an attempt to avoid anthropocentrism.
Why is an anthropomorphic tiger called a moreau and not just a tiger? Because a tiger is Panthera tigris, and a moreau is a genetically distinct human-animal amalgamation (or the descendant of one).
I think you're neglecting the most obvious reason people use words: to communicate something. "Youngling", which is not a new term really, is used in Star Wars media to refer to juveniles of a variety of species. Why not just use the word "child"? Well, because a child is a juvenile human being, with the timeframe and connotations that come with that. Using "youngling" makes the biological diversity immediately apparent to readers, and has the in-story effect of an attempt to avoid anthropocentrism.
Why is an anthropomorphic tiger called a moreau and not just a tiger? Because a tiger is Panthera tigris, and a moreau is a genetically distinct human-animal amalgamation (or the descendant of one).
Do neologisms always work? Are they always necessary? Probably not, but check out Merriam-Webster's lists of synonyms for "child".