First off, this is an excellent game that both kids and adults will enjoy - a great father-son diversion, but it can become somewhat addictive. You may soon find yourself adding new accounts to other devices(e.g. your wife’s phone).
If you refrain from spending real money on it(which I have), it’s also a lesson in careful planning and patience, as well as basic arithmetic. Clan vs Clan Wars promote teamwork, communication, and philanthropy.
The minimal violence isn’t any worse than what kids see in cartoons or Skylanders games. The only possible objectionable content is the open chat. There are two kinds of chat that you can toggle between: Global and Clan. You can report foul language in the Global chat and get people temporarily booted from the servers, but there’s no reporting function when chatting within your own clan. This makes the clan your child is in the real deciding factor in whether or not they should be playing the game. Fortunately, lots of clans have filled this need and you’ll find plenty of "clean" clans full of fathers and sons, clans which advertise "No cursing allowed"(and offenders are quickly given the boot), clans of school chums, etc. If your kid joins up with a clan of college-age gamers, though, look out! Keep an eye on the chat thread in your child’s clan(there’s a buffer of the last 100 messages) until you’re satisfied the members don’t curse like sailors, and this game will provide many hours of enjoyment for you both.