Honestly, it's not a stupid question at all. (That said, I never thought I'd have to google "child porn," much less at 10:30 on a Monday.)
While controversy about the legality of drawn child porn exists (some argue that it's a "victimless crime" because there are no performers), there seems to be very little controversy about plot vs. art style in determining "child-porn-iness."
There are consistent references to "characters that are underage or appear underage" in discussions of child porn. So, this'll be based on art style, whether or not the plot explains that they're of legal age.
To bring the discussion back down to a more legal (and local) baseline, there's RA 9775: An Act Defining the Crime of Child Pornography, Prescribing Penalties Therefor and for Other Purposes
In it, you have the following text:
For the purpose of this Act, a child shall also refer to:
(1) a person regardless of age who is presented, depicted or portrayed as a child as defined herein; and
(2) computer-generated, digitally or manually crafted images or graphics of a person who is represented or who is made to appear to be a child as defined herein.
So, legally speaking, local laws are pretty clear on what is and is not allowable.
(I apologize for the long, elaborate answer, but I thought it would be helpful.)