Dark souls is an incredible, fun, deep, rewarding game if you are the right person. If not, it will prove to be cruel, overly-punishing, hopeless torment.
The difficulty level is turned all the way up to 11 in this game, and will probably be remembered as the hardest game this year. If you or your child is easily angered from death/failure in games, than this is absolutely not for you/them. Regardless of the gamer's skill, it is almost a guaranteeing that they will die constantly from clever, brutal enemies, devious traps, and knights wielding giant hammers.
Onto the content:
Very brutal, blood spurts all around with plenty of disturbing, gory imagery. It is very realistic, and there are an incredibly high amount of ways to die other than "stabbed to death by sword." If the player is easily freighted by unexpected deaths, like being charred to a crisp by a dragon or being devoured by a living treasure chest, one should think twice before playing.
The ESRB rating is for "partial nudity," and this much is true. One of the boss fights displays a woman whose breasts are only covered by her long hair. Additionally, she has the torso of a giant spider beast. Other than that there is the occasional cleavage from a lady-character or shirtless guy, nothing one wouldn't see in a PG-13 movie.
A small issue, but an issue none the less. In the game's online function one can team up or attack other players in their own game-worlds. The only thing truly displayed is one's online screen name, thus allowing one to send messages to the account. Be sure to have a talk with your children to not give out any personal information before you let them play, though most evil-type players you meet online are only interested in killing your character and what they can gain from doing such.
As an additional point, the game has a very dark, hopeless, and off-and-on spiritual tone to it. Some players may find this very inviting and engaging to enter the game experience with such a strong emotional overset, where as other players could be disturbed emotionally from the constant stress on death and decay in the game environments. This game and its predecessor (Demon's Souls) are both well-known in the gaming world for having the ability to infringe on the gamer's mental well-being after extended playtime, however the amount that it does, and the intensity of its effect is reliant solely on the gamer themselves. This can be a good thing if the player commits themselves to the game to beat it, as the payoff and victory stand to be one of the most-satisfying victories in modern gaming. It would be a bad thing, of course, if the player is easily influenced by feelings up to the point that it would change their attitude from playing the game.
As a final note, Dark Souls is good if you like to be challenged, and to learn from your mistakes. The game is never unfair, only strict. One can think of it as a teacher, aiding the player to learn more and more about how to combat each enemy and situation as time, (and deaths) pass by.
Do some serious thinking before you let your children under 15 or so play.