TL;DR: Buy Minecraft for all your kids. Set up a server on your private LAN and play on that with them. The developer of Minecraft is inspiring and a great role model.
I'm 29 and I've had two runs where I've been obsessed I've spent hundreds of hours in Minecraft. It's been a long time since I was a kid and my daughter is only 9 months old, so I'm not as sensitive to what is "kid friendly" as I used to. That said, it's really hard for me to think of anything offensive or disturbing in Minecraft - in survival mode (I've only played this mode), monsters appear in the dark. They come out at night, or are found in underground caves. You can avoid them, or you can build a sword and armor and fight them when you run across them.
Most of the game is setting goals and achieving them. You have to start small (survive the night) but end up with huge goals (build a 1km minecart track, build a tower as high as it can go, redirect a lava flow, etc).
Playing alone is fun, playing on a private server with friends is even more fun. I highly suggest that parents set up a server on their private LAN and play on that with their kids. I expect that you'll actually look forward to coming home from work and seeing what your kids have done while you were gone. While they're asleep, you can decompress and build them something cool for them to see the next day. Your kids can have their friends bring over laptops or use spare computers and they can all play together.
The other aspect that I like about Minecraft is the developers and the community they have inspired. The "inventor" of Minecraft, who goes by the handle "Notch" is VERY open about how he made the game and comes across as a very nice person. Minecraft is another of those examples of an "overnight success" that came after years and years of failures, and he is very open about how hard he worked to get to where he is and doesn't try to take all the credit for his work. He recently split the millions of dollars that he made with his staff. Notch is also very big on "giving back to the community", he has donated large sums of money to every Humble Indie Bundle. You'd be hard pressed to find a game developer that would be as good a role model or as inspiring as Notch.