I think in general terms, what sexual abuse is is obvious. To me, anything sexual that happens between two people without consent is sexual abuse. It doesn't matter whether these two people are dating, married or strangers, when consent is missing it is abuse. Child sexual abuse is, in most cases, an adult abusing a child in a sexual way. I think it's pretty much clear that any sexual actions that involve children are illegal and wrong. But, what if these acts happen between two minors? Teenagers having consensual sex is not illegal because there is the argument that they are old enough to make these decisions. I think that is pretty much common sense.
However, what if there is a significant age difference - I think especially when you're a child a few years make a huge difference - and the younger child does not have the possibility to say no. Or doesn't know how to say no. Or doesn't understand what's going on. Where do you draw the line? Where is the line between 'children just being children and curious' and sexual abuse. I think most people remember 'playing doctor' in their early childhood and it is generally seen as normal curiosity about your own body. According to wikipedia: "Playing doctor is considered by most child psychologists as a normal step in childhood sexual development between the ages of approximately three and six years." So once the older child is significantly older than six, are any sexual acts child-on-child sexual abuse?
All of this has to do with psychiatry and emotions and personal experiences that cannot be depicted in an objective manner. I guess you'd have to look at every single case to figure out what it is. Is it really so important to be able to call it a certain name? It seems really important to me, but in the end what effects it has on a person and how they deal with it are probably way more important. Can you blame a child or an adolescent youth for initiating sexual behavior with a younger child when they might not even know or fully understand what they are doing? This kind of behavior certainly has to originate somewhere, but a curious child who grows up around older siblings will probably come across adult magazines/movies at some point.
I'm not sure if there's an answer to all of this. Obviously, exposing a child to sex in any way, shape or form too early isn't good. I'm not saying that you shouldn't talk to your child about how babies are made or that you should completely shelter your child from absolutely everything sexual, but with some common sense most adults know where to draw the line. A six-year-old certainly does not need a graphic description of how babies are made, much less pictures in adult magazines. A twelve-year-old can probably deal with information about sex, STD's and contraception in a meaningful way.
Anyways, just some things going through my head. Maybe I'll one day be able to write down my experiences and share them with this little blog of mine, but right now, I'm not. C'est la vie.