My daughter (8 years old) saw Janet's breast when she was watching the SuperBowl with her dad. Didn't appear to faze her a bit, but then we didn't make a fuss about it. She thought it was stupid. I imagine a 13 year old boy might have reacted a bit differently, but ultimately your kids take their cues from you the parent.
Personally I would rather take responsiblity for what my child watches than have a group of people censor things for me....but then I'm just a rebel