Selfhosted

OP, just wanted to say that involving your young nephew in constructive computer projects/activities is super cool. You get a Good Noodle star on your chart.

I'd argue home assistant with some smart LEDs and a few sensors would be great.

Having a bulb that let's you know the outside temperature/weather when you're getting dressed in the morning is neat. Having a dimming pattern for sleeping time. Tons of other really simple stuff available too.

I don't get why people continue to recommend Minecraft when there is the much better open-source Luanti project: https://www.luanti.org/

My kid is 9, and it's all Minecraft and I just installed a non federated synapse server so they can text family without having to deal with parental controls. At that age, I figure it's more about having fun, and if they learn typing skills on the way, big win

Hedy is an open source programming language that is broken into levels for easy learning. As you progress the language gains more capabilities, so they are never overwhelmed with too much

In contrast to block based languages like scratch its goal is to leave students ready to switch to Python by the end.

Each level has small tasks to complete so you can tackle it piece by piece and get a sense of progression.

https://hedy.org/

A Minecraft server is the classic.

Don't discount just putting together a basic webpage that can be accessed at home too- something he could put together in a basic HTML editor (drag and drop) and put his favorite things on or whatever he may be focusing on (cars, animals, space, you name it).

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? They should be familiar with the concept of a diary. 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...

I like the recommendations from other people as well. Sadly I don't know which kids programming language works best. I think I heard you can just go straight for Python as well. Not sure if that's true or what age group that applies to. It's a bit more involved to learn the syntax and why you need brackets around certain things etc but at least they get to learn the real deal and something properly useful. 7 might be a bit young, though. And there might be a language barrier. But that applies to all the computer stuff behind the scenes, unless you're a native English speaker.

Minecraft?

https://github.com/Lodestone-Team/lodestone?tab=readme-ov-file

HTML Website is cool. A child of that age might be able to write basic HTML. I wasn't much older when I made my own site

Minecraft server. Then Pufferpanel, then modding. Tons to learn.

Put it in a VM for him and take snapshots for easy recovery.

You can show them how to mod something they already play/like

Scratch, theres a foss alternative you can selfhost I think

Haha the one word he knows is #Minecraft so yeah it sounds like that will be task one. I can do that on a #Proxmox server I have.

Minecraft server.

I showed interest at around this age and my dad showed me CentOS and building basic webpages. I didn't take too much interest in that, but I asked him if we could build a Counter Strike server and he obliged. He's a nerd himself so we had a static IP for the server and everything. Worked well!

Anyway, I would recommend getting an old desktop and installing Ubuntu server or desktop edition with a desktop environment. Show him how to navigate the command line and what that means if you follow the file explorer at the same time. And then hosting very basic things(webpages, local game servers, etc.).

He might really latch onto it, or might not be interested whatsoever. I latched onto it, ended up building my own PCs soon after, and have my own homelab and I work as a full time Linux sysadmin now.