qualifies child .... there are degrees or grades of child - baby, toddler, young child, pre-pubescent child then teenager ... young child I'd guess 4-6 or thereabouts as opposed to 6-12; let's say child unqualified covers 3-12 - pretty arbitrary divisions 老鬼

You *can* simply say "child." But as the first answer points out, there are different phases of being a child, so sometimes "young child" can be helpful to refer to children of a specific age range. And it is arbitrary and also depends on who is speaking. For example, an 80-year-old man might refer to a 25-year-old woman as a "young child." But in most contexts, a 25-yeae-old would not be considered a "young child" or even a child. Of course, a person of any age, even 80,can be described as "childish" - means acting like a child. But, in general, the age ranges that the first answer gives for different stages of "childhood" are pretty good. Except I would say "infant" instead of "baby."