When I was a child, Thief: The Dark Project was new and exciting. It too was rated M for Mature, and I should not have played it at my young age, but I did, as I did with many other games which included Half-Life, Quake II, Unreal, Doom, and games of the like.
Thief: The Dark Project contained "Animated Blood and Violence". It was rated Mature because when you damaged someone a circle of red appeared on the floor. Times have changed, and those games would have likely been rated E10+ in today's world.
I am not here to tell you that you should not let your children play this game, I am here to say that you should just give it a good amount of thought, and hear me out. This game contains a decent amount of violence (albeit far from as intense as many other games on the market) and a good amount of strong language (which includes the "F" word, and nearly all words you can think of that are below it in the "badness level") fully depicted usage of drugs and alcohol, and sexuality that doesn't shy away from nudity.
In my opinion, the sexuality is at the same level as grade A pornography, which would include a mission that throws your character into a brothel sporting topless prostitutes, and peepholes you can look through (and might need to look through to accomplish an objective) that show these prostitutes with their clients. One shows a prostitute seducing a man by simply talking graphically dirty to him, all while being topless. Two others show a woman sexually abusing a man for his pleasure and a completely naked woman thrusting her hips on top of a completely naked man. Even if you were to refuse looking in these peepholes, you would more than likely stumble across a topless prostitute beforehand, and you would hear lovely child-friendly things like climaxes and dirty talk.
Although killing is just as immoral as the sexuality in this game, it is a goal of the game to not engage in conflict. The game encourages players not to kill for the sake of both skill and a better score.
But, in the end the unavoidable profanity, drug and alcohol use, along with the graphic sex and potential violence might be a bit much for absolutely any person that limits themselves from witnessing these things.
My end point?
The quality of this game isn't even all that high. Better games exist in its genre. And the amount of both adult and pornographic content locked into the plot of this game has to leave someone asking: "Is this really worth it?"
My advice is to save your money for a more tasteful game that is both less unreasonably inappropriate and higher in quality.
And to the parents out there... think twice.