Reading through the other thread, most of the confusion comes from the blurry line of the first two subtropes.
The third is always explicit in the story, and definitely needs to be its own trope, it should not be lumped in with the other two. Also the Cable image is perfect for that trope.
Like I said in the other thread, there are really three things going on here, not counting the time travel/magic/whatever situations:
A) Young girl with a child. ie: 16 year old with an infant, or whatever. This is pretty much always explicit, and generally has fairly significant implications in the story
B) A woman who had a child when she was unusually young, but is now an adult. ie: 32 year old woman with a 16 year old child
C) A woman who had her child at a perfectly reasonable age, but due to art style, or just aging nicely, she looks way younger than she should. ie: 40 year old woman with a 16 year old child, but could pass as the child's older sister
I'd say that A) is a distinct trope, while B) and C) generally are both used for "hot mom" type stuff.