In the US of A, the definition is embodied in Ttile 18 of the US Code:

"TITLE 18 > PART I > CHAPTER 110 > § 2256

§ 2256. Definitions for chapter
 
For the purposes of this chapter, the term—
(1) “minor” means any person under the age of eighteen years;
(2)
(A) Except as provided in subparagraph (B), “sexually explicit conduct” means actual or simulated—
(i) sexual intercourse, including genital-genital, oral-genital, anal-genital, or oral-anal, whether between persons of the same or opposite sex;
(ii) bestiality;
(iii) masturbation;
(iv) sadistic or masochistic abuse; or
(v) lascivious exhibition of the genitals or pubic area of any person;

(B) For purposes of subsection 8(B) [1] of this section, “sexually explicit conduct” means—
(i) graphic sexual intercourse, including genital-genital, oral-genital, anal-genital, or oral-anal, whether between persons of the same or opposite sex, or lascivious simulated sexual intercourse where the genitals, breast, or pubic area of any person is exhibited;
(ii) graphic or lascivious simulated;
(I) bestiality;
(II) masturbation; or
(III) sadistic or masochistic abuse; or
(iii) graphic or simulated lascivious exhibition of the genitals or pubic area of any person"

Of course, this refers to actual children, not "pixels," see: http://archives.cnn.com/2002/LAW/04/16/scotus.virtual.child.porn/

This was the pre Roberts/Alito court but since Justice O'Connor voted with the minority and Justice Thomas agreed with the majority, that opinion might well be upheld even today. 

'Rosity's rules clearly ban most and perhpas all of the proscribed depictions, even involving "adult" models.  In choosing to ban all depictions of nude "children," they are steering clear probably the most perilous area that exists.  Porn aside, many people, especially in the United States, feel strongly that any depiction of underage nudity is evil, perverted, etc.  The constituency who would defend the validity of such  on artistic or fee speech grounds is vanishing small by comparison. 

Even many of those who might agree are reluctant to stand up for fear of being branded as sickos.  On very rare occasions, people with deep pockets say "enough."  Barnes & Noble defended their right to sell photography books by Hamilton, Sturges, etc. and won.  The movie The Tin Drum  was ruled not to be child pornography after Christian fundamentalists in Oklahoma City attacked the film.

'Rosity is only doing what makes sense to them and protects their interests.  You might find some fault with them if you believe that they are really about art but does anyone really believe that?  The only dragon slayer here is Vicki and she has her hands full with people who want her to put clothes on.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken