There have been kids prosecuted for sending pictures of themselves out over the internet (despite what I think is a good argument that the laws were not meant to prosecute the actual, underage subject of such pictures).
Realistically, prosecutors aren't likely to go after the kids involved. There are plenty of real kiddie-porn producers that are higher on the list.
And yet, there's a real need to prevent this stuff from becoming generally available, because there is a legitimate public interest in keeping it out of the hands of perverts. It's a tough nut to crack.
In reality what has happened is that legislators assumed that underage teenagers would be moral enough not to send naked pictures of themselves to others. I probably would have made the same assumption. In hindsight that is a mistake and there is now a gap in the law. The problem is, how do you update the law to change the situation? If an underage teen with a grudge can send her teacher an email with a naked picture and get him put away, that doesn't seem right, but if he is in possession of the picture, it doesn't seem right either. Underage kids possessing naked pictures of their peers doesn't seem so bad until fathers get a hold of them (realistically, women can probably get away with possessing any amount of child porn).
So what is the answer? Let things continue on a case-by-case basis until our populace is used to the idea of underage teens media-messaging nudie-photos of themselves to each other? Trying to stamp out the practice by prosecuting those who do?
Realistically, they are. Once moral panic sets in, reason goes out the window of zero tolerance.