MineCraft has a huge following and my grandchildren play it whenever allowed.
You arrive in a virtual world and have to fend for yourself, build a shelter, find food and make ever more sophisticated tools. You live by mining, farming and bartering.
It provides an opportunity to explore, build and discover things for yourself and every aspect of the game can be modded.
Although it is intended for playing on-line as part of large community, you can download the server version and host that on your own machine and the grandchildren then have a private world they can log into across the LAN, or even from home.
There plenty of on-line help, especially on Youtube.
It costs a one off £16.99 to join.
It is entirely untrue that I waste hours on Minecraft, but my enchanted diamond suit of armour is an incredible thing.