MS Word can be lots of fun and is easy to get started with. For sure let them design their own birthday invitations and so on. I had a lot of fun as a kid play

MS Word can be lots of fun and is easy to get started with. For sure let them design their own birthday invitations and so on. I had a lot of fun as a kid playing Minecraft and building Redstone contraptions. Minecraft does invite tinkering with texture packs, mods etc. So I'd say just go with a basic windows install on a normal PC.