First, let's be clear about the rating system. They are not a definitive guideline to what should and should not be played at certain ages. It is your choice on how strictly you wish to follow them of course, but always make your own decisions on whether it's ok or not. The only thing that has really changed over the years is that people have become much more sensitive to certain topics being taught to kids. You can let your kids plan any type of game you wish. As long as you can properly teach them right from wrong and how to separate the real world from the game world, they will be fine. There is a slight issue with more violent game in which it can possibly desensitize your kids to violence, but again with moderation and proper teaching this can be minimalized. In the case of splatoon, it has small amounts of cartoon violence. But other than that, it's a very simple game. Just shoot globs of paint around you and at others. I think it's a game that could easily be played by all ages. It's no more violent than children having a snowball fight.

Wait, what? I would let a five year old play that game. That's like the best thing about the game. It allows kids to play a fun shooter

https://m.youtube.com/watch?v=mZjS6YMb7C4 You're a kid you're a squid you're a kid you're a squid you're a kid you're a squid

Go right ahead. Splatoon is meant to be a children's game.