This would have no legal basis whatsoever. First, thought crime isn't something envisioned in modern legal systems. Pedophilia isn't accepted because childs can't consent and consequentially they can't have sex or anything (with exceptions of both minors and whatever). There is no consent dilemma in cartoons. Banning it would be more alike at banning omosexuality, where adults do things in full consent and it's still banned because it goes against "morality". The gateway argument make no sense either; it made no sense for drugs, it made no sense for games that promote killings, and it make no sense here.
Second, you have no ways to define a child in stylized drawings. What define a child? Being short? I just give it long ears and call it an halfling. Pubes? Plenty of adults shave. Small tits? Plenty of women have no tits. Even if such a ban went in effect, the only thing that would change is that everything would be considered done by adults cosplaying. Sorta like "this work is fiction" and whatever.
There's way more ground to ban actual children models. That has proven disastrously effects on a lot of children's psiche, and i don't think a parent giving consent is a strong enough rebuttal, since we ban most dangerous things from childrens no matter the opinion of the parent. There's a reason that kind of thing is not done anywhere else in the world but japan afaik.