Outside of the first game, the Yakuza series does not have any swear words in English and although I am not well versed in the same topic for Japanese, I know it is low. Even then, usually the games are text based. As for the rest of the series (except one thing near the end of Yakuza 4 and obviously OF THE END) the series is not dark or out of line at all. If anything I would argue Star Wars is worse for children with how Anakin changes from that cute kid to the meanest thing in the galaxy compared to how the playable characters in the Yakuza series are some of the most respectful and dignified characters SEGA has ever made.
Although at the end of the day it is up to your judgment, Emmett. I would never tell anyone how they should raise their children, I just think establishing all of what you hide from them should not be based what is said on the box.
I mean, what makes something like Halo - An M rated series, outside of Wars - any worse than laser tag with toy guns? 20+ year old games have no consistent rating, so in those cases you would have to make your own choices. I mean if you think about it, Revenge of Shinobi is pretty dark, would you let your kid see/play that? Movies are another thing, what was considered an R movie back in the early 1980s is now considered PG.
I understand why you would want to do this. While I am not a father myself, I have always wanted to be one and have had the topic on my mind a lot. I would obviously never let them know games like Gears of War or Bayonetta exist, but stuff like Yakuza and Halo is not going to scar anyone.