Tenchu Z is rated M (Mature, 17+) for Blood, Partial Nudity and Violence. It's a ninja game so there's a good amount of stabbing, with blood.The game is one player, or 2-4 playing in online/co-op modes.
Our family has been playing Tenchu for years, and I have to admit, even our 6 year old plays Tenchu Z on practice. In certain modes, it's like any traditional fighting game where you battle, you can buy weapons and armor and assign them, etc.
Adults will likely choose to complete missions, and you can come across things that aren't so child-friendly. I think the key to this game is like any other, supervise your children while they play.
While this game advertises 50 new missions, including online co-op available on the 360, it's not all its cracked up to be. The graphics aren't impressive. The levels seem to be much of the same thing over and again. They managed to fit in some of the old staples we know and love but overall the game didn't feel well done. It doesn't have that "replay" factor the other games have.
You do have the ability to totally trick your ninja out, even with custom moves, but it wasn't enough to keep me interested long-term.
If you're a Tenchu fan, rent this game and get it out of your system. I wouldn't buy it unless you make it a point to buy all the games in a particular series.