Reason: Child currently covers a large age range (3-12) that includes a wide swath of varying physical characteristics. A "child" of 4 has extremely different physical characteristics from a "child" of 12. There currently is no way to specifically search for the 10-12 age range (except for the tween tag, which is currently affixed to 17 pictures, so I'm pretty sure it's not official), and yet there is a Baby, Toddler, and Teen tag, all of which cover specific age ranges with specific physical characteristics.
Updated