The United Kingdom's Coroners and Justice Act 2009 (part 2, chapter 2) makes possession of images portraying fictional child abuse punishable by law. Wikipedia has a summary, but here's a snip from the legalese:

(5) “Child”, subject to subsection (6), means a person under the age of 18.

(6) Where an image shows a person the image is to be treated as an image of a child if—

 (a) the impression conveyed by the image is that the person shown is a child, or

 (b) the predominant impression conveyed is that the person shown is a child despite the fact that some of the physical characteristics shown are not those of a child.

Of particular significance is the conviction of anime fan Robul Hoque whose cartoons the court found to be infringing. As summarised by The Mirror's Gareth Lightfoot:

Despite being cartoons, they were classified as prohibited images as they depicted young girls, some in school uniforms, some exposing themselves or taking part in sexual activity.

The lawmakers responsible assert that the law helps to protect children against abuse. I suspect that their motivation stems from a wrong working hypothesis of "fantasy leads to action". Regarding other human sexual behaviour, the contrary is supported by evidence that widespread access to porn decreases incidence of rape and banning brothels increases incidence of rape.

Does similar evidence exist for whether such a ban protects children from abuse?