Seems simple on the surface but there are some interesting ramifications in all this. In the US, if you are over eighteen and knowingly shoot and post nude or partially nude images of someone under eighteen, you can and most likely will be prosecuted for child pornography. This came up when some of my students noticed there were nudes included in the AICE (Cambridge CIE) portfolios presented from schools in Australia/NZ/and some parts of Europe where the rules are different.
My instructions were to ensure the shootee was over 18 - PERIOD and even at that to make sure they had written permission from both the parent and the model. Anyting under 17 and I would pass it on to the School Resource Deputy and suggest prosecution.
Sexting gets kids arrested al lthe time. It's not nearly as prevelent as it was a few years ago.
Art photography, nude or otherwise is wonderful art, but it's a dicey interpretation when it comes to kids. My advice is DON'T!