One of the highest pedigrees in gaming. Difficulty and content-wise absolutely not for children.
Demon's souls, as fearsome-sounding a name it is for a video game, fits it rather well. This will probably be the hardest video game you have ever played if you have joined the gaming horde in recent years, and you will be tested brutally. This is the game to hand your friend at a party whilst they boast about their immense video-gaming prowess; you can expect their attitude to quickly change when Demon's Souls is brought up. You will be humiliated by death constantly, and almost every time, you will lose your souls (experience for leveling up your character) and placement in the area. That said, when you die, you go back to the start. Thankfully Demon's souls has mercy in the fact that it is very coherent, and you can understand why you die in almost every situation. It is all up to a player to throw the nails in their coffin. When you start out in the game I can near guarantee you that you or the one you buy the game for will die again and again. It will be frustrating experience, at first.
Then the player realizes, once their personal skill rises, that this is one of the most satisfying video games they have ever played. Every act you do is so soul-crushingly hard that if you are successful, the feeling of payoff is like no other in any game in recent memory.
Past the game itself, it also has some overtones and messages that, if the gamer is unprepared, could actually infringe on their mental health. The world is incredibly dark. Not like a "shoot em up extreme" dark, but more like a "you have no hope" dark. You character is surrounded by disgusting half-humans, killer beasts, and things so horrifying they would be hard to describe in words. Demon's souls can "grip" a person in this manner, up to the point where it can infringe on the player's mental health. So if you or your kid plays this game be sure to have them take breaks often.