Even though most fanfic is unlicensed, FanFiction.Net attempts to implement digital rights management, such as disabling text selection (which can be thwarted with userscripts or NoScript for Firefox) and blocking downloader software such as FLAG.
Great site for sharing your home-written fanfiction, but parents exercise good judgement when it comes to kids, because there is mature content. The site does provide an automatic filter, but it only takes a few clicks to disable.
Fanfiction, or more affectionaly known as FFNET, is a major source of MILLIONS of fanfictions. The only thing is that they don't monitor what goes through and the only way they know there is adult (MA) content is if some one reports the story as adult content.
There is a rating system, so to get to it the kids have to ask for it, so really it's their fault if they stumble upon something they shouldn't see, but still be careful.
This site is awesome. I've been a member at FFN ever since I began using the internet, and it's still a great source for fanfics great and small. However, there is some adult content in some of the stories on the site. There's definitely some fics I've written that I would not want kids to read. This site DOES have a required rating system - to make a new story and publish it, you must rate all fanfics appropriately, and failure to do so means that the story gets taken down. The rating system makes it easy for readers to know what they're getting into, and many authors also put disclaimers and descriptors into the story summary area so the reader knows what's in the story. Therefore, this site's actually pretty safe, provided that younger readers have someone who's aware of the rating system and of fanfic in general to guide them to appropriate stories.