I'm not a parent, thankfully; but if I were, I'm not sure I'd be entirely comfortable with the idea of other parents seeing my child naked. Even if I don't sign my child's account up for this scanning thing, the recipient could well be, and they won't be warned about that before sending it.
This is an abusive parent's wet dream waiting to happen, and it disgusts me. Minors should be free to explore their sexuality with their peers. It's how we grow as people.
EDIT: Upon further thought; how is this NOT illegal? How else can this be framed than "Apple knowingly and willingly assists in the dissemination of child pornography into the hands of adults"?