If you're just downloading and looking at porn, no problem. It only becomes an issue if you're sharing porn via Messages or storing it in iCloud. And to be fair, I don't think they're alerted to the nature of the pornography, so you might be able to avoid being outed even if you're sharing porn (or having porn shared with you).

Edit: I'm wrong in one respect: if the kid under 13 chooses to send a message with an explicit image despite being warned via notification, the image will be saved to a parental controls section. This won't happen for children >= 13.