Under the hood, Minetest is way cooler than it looks: It actually implements a generalized 3D cellular automaton framework in which each cube in the world is driven by a small Lua program – which makes it much easier to mod than Minecraft. Many people like the Minecraft-style mod, but there exists, for example, an implementation of Wireworld: https://en.wikipedia.org/wiki/Wireworld

Disclosure: I was an early Minetest contributor when you still had to provide C++ patches to add new features (2011) and drew some textures.

I'm doing a programming workshop for kids using Minecraft and Python and looked at alternatives so not everyone would need a copy of Minecraft (for 24€ or so).

My experience with Minetest is that it's not in a good shape. I stumbled over several serious bugs (I reported them but they weren't fixed quickly) and gave up on it. Usability also wasn't very good. They really need a lot more polish.

Too bad really. It has the potential to be great for sure.