I think educational activities work best once they have some application to someones life. So it’d be something within the realm of a 7yo. And it’s not fun unless there’s a sense of achievement every now and then, along with all the stuff to learn. So probably not too steep of a learning curve.

Sadly they discontinued Lego Mindstorms. I think robotics is a great hands-on topic. People can grasp what they’re currently doing, why they do it, and what it’s good for. It has a tactile aspect, so you’ll train dexterity as well and gently connect the physical realm with the maths.

But other than that, I bet there’s a lot of things you can try. Design a website (and deploy a small webserver). Maybe some easy to use photo gallery if they have a tablet or camera. Maybe a Wordpress for them to write a Blog? Kids love Minecraft, so maybe a Luanti server if you’re into Free Software. But learn how to add NPCs and animals, that is (or used to be?) a complicated process in Luanti and the world feels boring and empty without. A chat server to their loved ones could motivate them to read and write text (messages). Or skip the selfhosting aspect and do the kids games available for Linux. Paint, LibreOffice…