I've been on the site since the beta period and seen it grow and change and I'll say that if I were a parent, I would keep a very close eye on what happens on the site. While the artwork and content is good and the games are simple and would bore anyone that's not enthralled with the Potter series. This unfortunately doesn't redeem all its shortcomings as to safety. I've seen a few role plays as they're called that seem overly violent, with one going over torturing an individual and disposing a corpse, whereas the other was simulating sexual intercourse. While both of these were not on the main message boards on the site (the great hall, common rooms, dueling corridor and potions area) while trying to go deeper into the story and see more content, it would be easy for someone to accidentally stumble upon this content, with some of it being on a page for the house founders, linked directly to the main message board (the great hall).
Really, it is almost a given for the internet, but given that some users do seem to take pains to get around the reporting function, and that at times, reported comments with truly offensive content are reposted by Pottermore's shoddy moderation, it seems to be a wise idea to supervise any time spent on the site, or if your children must, to keep it to the main areas, and moments, where most users seem to police themselves.