Be careful on fansites, otherwise great for kids
The website itself is completely safe for children and the comments from others are generally supportive.
However, be wary of fansites because the website has also attracted a lot of 12-18 year old teens who create these. The main one is on Wikia, which may not be the best environment for kids because, well, Wikia.
Also, Skype calls with community members are extremely encouraged among the well-known users. As for these kids themselves, they're usually just as supportive, but they can be immature at best and drama queens at the worst. It's also really easy to become well-known, pretty much all one would have to do is get VIP (which is actually pretty cheap) make a lot of videos and join a fansite.
TL;DR Good site for kids, may taste 15 minutes of fame a little early.
What other families should know
Safety and privacy concerns