Luanti (Formerly Minetest)

I have a human child that has created a rather awesome Minecraft world.

Now there are several reasons why it would be good to start using something based on Luanti.
However, the world they have been creating is something we would really not want to miss, even though it's not even one year old.

They are playing in creative mode, and we agreed that it's enough if all of the terrain near the home house, the house itself, plus the tunnels dug into that terrain would be preserved. We are able to rebuild all the minetracks and doors and windows and place new animals, if needed. But at least a skeleton of their world would be important to preserve.

So, is there a way to convert our Minecraft world to something playable on Luanti? And what game running on Luanti would you people recommend for us to use for this purpose?

And secondarily, if we need to just start from scratch, which Luanti-game would be best for a young child wanting to play more or less akin to how you play Minecraft in creative mode? Being able to place objects freely while flying freely, and beig able to populate the world with animals – at least cats and/or dogs – would be very important for them, but almost anything else can be whatever it is. It does not need to be the same as Minecraft, it just needs to be able to approximately fulfill the same purpose for a human child.

I'm running this on Ubuntu and if any Windows or Mac based software should be needed, I do know people who can let me temporarily use their computer for that. But the actual playing will happen on Ubuntu.