Well sure ... but apparently it's the kids themselves who are posting the pictures and videos. Can you call them "criminals" in the same sense as a NAMBLA mouth-breather?
Possession of any pictures of nude children is a felony. The fact that they are juveniles does not play any role in determining guilt, though it might affect the punishment. But, remember, many teens have become convicted felons for their dealings with marijuana.
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.
The law allows for prosecution for mere posession.