It is simply unbelievable that the myth of sex being dangerous to children still persists!
The reality is, that if the children are too young, they neither understand nor care about sexual imagery in any form. If they're old enough, they're probably already seeking it out on their own. Massive studies in countries where hardcore porn has been available in shop windows and magazine racks in supermarkets for decades have shown that is has almost no effect what so ever on children and young adults - the only possible related effect seems to be a decrease in teenage pregnancy but it can be completely unrelated.
I think the danger posed by children having access to sexual imagery is that it can help de-tabuify sex, in particular the deviations that can scare teenagers into various closets. They need to find - as early as possible - that they're not alone. If they are able to look it up, they'll find that there are many others with the same kinks and desires as themselves, and they'll be able to find a local group where they can enjoy being like everybody else and feel normal.
Unfortunately almost all religions insists on strictly regulating everything relating to sex, and especially children and sex. The fact is that most people (both sexes) will tell you that their first sexual fantasy occurred as early as in in their preteens and the natural curiosity about this usually led them to seek out books and pictures, but for perhaps all their teens, access was hard, especially if the taste deviated from the mainstream. Even harder was access to knowledge about their brand of sex and sexual shame - a mainstay of many religions - can become fuel for runaway perversions and lay the foundation of abuse of the next generation.
The best solution is of course to rid of all the religious junk thought and to move on with a life of knowledge and reason, free from the scourge of religion.