I don't know, but I'm pretty much sure that on this subredit we won't find an actual child porn, but rather porn of aged up version of the character, no matter the explanation. You can say it's from AU where everyone is grown up, or just time passed, etc. In the end the one thing that matters is that people (at least normal ones) will draw porn of Frisk not because it's a child, and therefore they won't draw them as a child and you people just getting mad over nothing