First of all, the "free" part of the name does not imply "no money required". It implies "do what you want". Subscriptions are practically mandatory for leveling your professions, and the Cash Shop will sell all sorts of goodies for money. As for content, there isn't all that much in game, but, as in all games, there's a reason why ESRB doesn't rate online interactions. The SOE crowd in general (Freerealms, DC Universe Online, Clone Wars Adventures, etc) tends to be unhelpful and best and inappropriate at worst. Always keep an eye on the screen while your child plays. Violence isn't too bad, mostly wacky fantasy stuff. Battles come in minigames and, more recently, the Sunstone Valley section of the map. In combat, your characters uses swords, scythes, hammers, bows, axes and magic spells to fight silly opponents like ninjas, giant insects and crabs, and during it's anniversary event, a giant animated slice of cake. Players can obtain seasonal weapons as well, such as balloon weapons during the aforementioned anniversary. Although there is no explicit sexual behavior in game, players may speak how they wish. There used to be an exploit which allowed players to strip down to their underwear, though they may have patched it up. No foul language in game, but again, the community. A town called Chugalug (I think, it's been a while) is populated by creatures who are seemingly drunk, and the town's banner is an ale mug.
Aside from that, not too much to speak of. The game gets dull after a while, as gear differences are barely noticable, and there isn't really an endgame other than getting the flashier gear for level 20 jobs. Most likely, your kid can handle it, but it most likely won't hold their interest for too long. You never know, though.