I have to say, in response to the original review, the movie was more or less supposed to be a stand-alone type of film. That's why the "supporting" characters are more detailed; It's made to get those who saw the first Fullmetal Alchemist animation and even those who haven't to watch the newest series, Fullmetal Alchemist Brotherhood. Even watching it after Brotherhood, the movie got me back into Fullmetal Alchemist Brotherhood, and I ended up watching it over again. Of course, a little bit of prior knowledge doesn't hurt, but it's not exactly necessary; I took a friend who didn't know anything about Fullmetal and he understood most of what was going on thanks to the movie's separate story. Some of the jokes that reference to the original animation series, however, flew right over his head and needed explanation. Other than those, this movie's great for those who at least know Ed and Al are brothers who broke the laws of human transmutation (They tried to bring back their dead mother), alchemy involves the law of equivalent exchange, and the Philosopher Stone allows them to bypass said law.
In reference to the violence, yes, there is a LOT of blood. But then again, we have to keep in mind the original anime had blood and gore too. If you kid's used to blood as most children above 10 or so are, this will be no worse than anything else they've seen. I don't know your child or what they've seen; so parents should make the judgement calls themselves.