Be sure to take a moment to reboot into desktop, and search the built in Linux app store for free games. There’s some absolute gems.

Edit: And after installing, right click the game on the desktop or menu, and choose “Add To Steam”, to make it show up in the usual Steam interface.

I used to always use Minecraft for this. Sure, they can’t do everything immediately, but I put the game on peaceful and let them explore at their own pace.

I say “used to” because Luanti (formerly MineTest, an open source Minecraft Clone*) is finished and free.

(Okay, Luanti is a lot more than a Minecraft clone. But for this discussion that’s all one needs to know.)

Nice. Minecraft used to be my go to answer for a first WASD game, too.

Lately, I recommend Mineclonia on Luanti, because it’s free and has stronger optimizations supporting weak laptops and big multiplayer servers. (It’s a popular Free Open Source Minecraft Clone).