In general, yes the site is fine. We moderate the Q&A pretty well. But we can't protect you from anyone swearing or talking about mature subjects on the chat room, so you may want to avoid the chat room.
Actually, I will soon be adding some Terms & Conditions to the site that say anyone under 13 (actually, anyone in general) uses the site at their own risk. There is a legal thing that means you must have specific measures in place if users are under 13 and I don't have the time, resources or effort to do that.