BEFORE you allow your child to join Pottermore, you need to know a few things both good and bad.
There are a lot of people on Pottermore right now but the only way to communicate with each other is in full view in the House Common Rooms, in full view in the Great Hall, and in full view in the comment sections at the end of each chapter. There is no personal/private messaging. Every word your child types is in full view of everyone else, and you can at least hope that there are responsible people present to report any problems. That's the good news.
The bad news is, that apparently Pottermore doesn't always give a darn about what others post that your child may read. It took many people several weeks of reporting via the report buttons, reporting via the Beta links, reporting via the Help link, sending in screen shots of R- rated role play in the Great Hall to Pottermore, and eventually sending those screen shots around to publishers, agents, and other Harry Potter fan sites before someone yanked Pottermore's chain hard enough that a scant handful of accounts were temporarily (a week or two) banned from access. During those weeks when those accounts were banned, the reprimanded individuals simply put into use all the extra accounts they made during the original "Quill Challenge" for Beta users and passed them around to their banned buddies, so they never actually lost access. Those accounts were used to continue their role play, although at least milder than the graphic rape-murders which were originally reported, and to throw screaming hissy fits about their right to free speech in the common rooms and Great Hall.
We knew Pottermore would have to be more specific about the rules governing member posts. When they published their new Terms and Conditions, they were more specific alright. They added legal protections for themselves all over the place which are simply: We try, but we can't control what your child sees here. Then they cap it all off with this statement "To be clear, each user, acts on his/her own behalf at all times and does not act as our representative or agent in any way. We do not endorse nor are we responsible for any of the actions of any Pottermore user."
What this boils down to is that in spite of repeated pleas to hire actual people to moderate the user comments, Pottermore still insists upon using an automated system. This automated system of moderation will prevent you from telling someone the highest possible score in a duel because numbers are not allowed lest your child give out their phone number, but the system will not prevent role players from passionately or forcefully "unicorning" one another in the Great Hall. The automated system only looks for words, not context, so when a couple makes out and lays down for a hearty unicorn, that's a-okay because unicorns are just pretty creatures right? Everyone watching knows what's going on, but the automated system is clueless.
I'm not saying do not let your child join Pottermore. I'm just saying that Pottermore has refused to do what it takes to monitor what your child sees. Pottermore, to all appearances, simply does not wish to spend the money for real "Prefects" who can implement instant 30 minutes bans, remove posts, and monitor user generated content. Have a talk with your kids before you let them join. I know at least some parents do that. Most, however, are clueless and others simply have a blind faith in anything related to Harry Potter.
This is the first time I have had to adamantly say, you cannot sit back and simply trust the Harry Potter name. I hate the fact that I have to say that, because I am a great fan of the books and the author, but in the case of Pottermore, it is the absolute truth. The muggles at Pottermore need to read the books. The books faithfully show that what is right is seldom easy. You cannot trust Pottermore to do what is right. If their behavior over the 8 month Beta period is any indication, Pottermore will only do what is easy.