Is child nudity illegal

Yes, they are. It's called child pornography. You could go to jail for it.

As the previous commented mentioned, yes nudist teen pics (depending on what age defines a minor in the place you currently reside) can be a ticket to jail. However photos of nudism, or naturalism, is quite ambiguous a in concern to being a form of child abuse. From what I have read it will all depend on the belief of the people in the court your sent to. Sadly (at least in my opinion) it seems that the laws in question can be still be interpreted in different ways. This means that if one owns an image that is not depicting physical sexual acts being done to a minor and only show adults and/or minors behaving like other people would (only without clothes), you still have the chance to be sent to jail.