I don't think they need any books about it at that age.As long as they know their body parts in general,not specifically sexually, and what they're for that's all they need to know.It's not something that enters the thoughts of many a five year old unless someone put that thought there.By senior school age,of course they'll be discussing it with their friends,I know we did lots of did you know,noway that's so gross,can't be true,and not my parents type discussions,and from evesdropping on my own kids they're doing the same,and there needs to be some kind of education because the children won't get it right,but this is going too far and too young.
Let them find out in their own time in their own way,and answer questions as they come from the child.
Sexual positions is something you work out yourself with your partner as an adult in a healthy relationship.
Hope it's one of those cases of the media getting hold of a story and making it sound a lot worse than it is,because it's certainly not something I'd be happy my children's school introducing them too.
