I actually did this for about 5 years (5 years ago), specifically making games in Scratch. I’m wary about sharing too much personal information in a comment thread so feel free to message me if you’d like to discuss further, I’d be happy to share context that might be relevant.

Here’s a few key things I learned.

• Get them to interaction as soon as possible. It’s better to start with a template game that is “playable” but has room for modification e.g. movement speed
• Age is little indicator of aptitude, I saw 5 year olds speed past kids twice their age
• Set expectations for what can be done: in scratch; in the time you have; by a single kid. They don’t necessarily know they won’t be able to make a massive 3D open world game
• Few kids I saw were excited by programming in and of itself but plenty took to it as a means to an end. They might be way more into making a cool animation and they’ll learn how to programme an animation. You have to figure out what they want to do and then show them how to do it by programming
• Not specific to programming but kids have lives and you have to be prepared to deal with that. They will not all come to you ready to learn, they might be dealing with any number of other things, and they won’t leave you unaffected. This can be as difficult as it is rewarding but it’s ultimately why I left working with kids. I don’t want to scare you off but I saw a lot of adults completely unprepared to be in a child’s life and you very well could be. I’d even go so far as to recommend seeing if you could get someone involved who works with children professionally. I have a lot more on this specifically but down want to pour too much cold water on you.

Edit: I forgot to add, I did this for a living, not a volunteer

The whole scratch thing is extremely well put together and is easy to teach. Small chunks, fast results, forgiving environment. It gets kids thinking programmatically without even trying and for those that take to it, it is trivial to say “here are some code words, write this in code”

The only challenges are differences in ability which in a mixed class of 8-12 yo might be significant, so it would work best if you segment them into those that grasp it quickly and those that take a little longer (not necessarily age)

The hard bit with teaching kids anything in keeping them tuned in, and a segment approach can help.

Get some robots. Big marketing thing and another point of fun and learning. And the results are visible IRL.

Scratch is great but make sure to let them know that they can see the actual code that runs underneath the drag-drop UI.

I can also recommend hourofcode.com where there are a ton of good tutorials ranging from scratch-level coding in a game setup (e.g. minecraft) to actual coding in python.

Check out https://github.com/HollyAdele/awesome-programming-for-kids

This was a cool talk on teaching programming gradually (with a lang called hedy) at last year’s strangeloop: https://youtu.be/fmF7HpU_-9k - might be some useful takeaways for you in there

There's a minecraft mod called ComputerCraft Edu which is lovely and works on older versions of Minecraft if you're willing to set that up!

When they "get" it, it also has Lua in addition the the graphical scratch nodes

When I was a kid Lego mindstorm was the shit but I don't know if Lego even sells that anymore. But my advice is find some robot kit with a building block scripting language.

Alternatively download unreal engine 4, learn how to use blueprints and show them how to make simple games, as that is another "building block" scripting language.

Edit: oh, not "from scratch", "using Scratch" haha oops. My bad.

I've used scratch to introduce kids (6~10) to programming. It works quite well IMO. They had a laptop with windows. I recommend a touch screen if possible, especially for younger kids. Though at 8~12yo that should not be as much of an issue.

I used it with the microbit from the BBC. While not required, a dedicated piece of hardware makes it much more interactive and fun, for a basic introduction. Basically, the microbit can be turned into a remote control for your characters in scratch for example.

Though, kids get fond of the ability to create pre-programmed scenes. That are not very logic intensive, more like an animated movie. And since they can add their own drawings and voice, they can get very engaged on this sole basis. So the microbit is not required at all.

Though if you want to use it, Microsoft has its own scratch for microbit that is more annoying to use IMO (you need to flash the program every time, which is not easy for younger kids that have trouble with the mouse), but it unlocks all the capabilities of the microbit for even more interactive applications. You can make them communicates through a basic protocol over 2.4GHz radio, control led strips or even robots for example (though the robots are far from cheap for what they are 😕).

Both scratch and makecode (the links mentioned above) have plenty of resources if you want to get a lab going. Personally, I would set my expectation fairly low and plan for many additional small features that kids that are really interested could implement on their own. In my experience, some kids will not be interested at all, not until they see a feature they want to interact with at least. Others will try to see what they can do by themselves, before the lab even begin. But usually, the older they get, the less likely they are to experiment by themselves and they'd rather wait for instructions. Which is a shame, but that's how it is I guess.

Also, try to make sure they can continue their work from home. Scratch is available on many platforms (though makecode sucks on Android last time I checked) and is trivial to get up and running. That said, importing a project is another matter for kids barely familiar with computers, which is why I would distribute a document aimed at their parent to get them set up.

I genuinely don't know if scratch is the right choice or a simple text based language would be better, especially for the older kids. Just from my personal experience, I started programming in BASIC at 12 and don't think I would have had as much fun and continued programming if i had used scratch instead.

Try https://code.org. It's free and really well done with a low barrier of entry. There are different types of tasks for various ages.

Coder dojo have lots of age suitable things to get groups like that started. It's focused on scratch to get going but moves into other things as they progress.

I taught at our local one for a while and it was great having lessons all planned out for you based on age and experience.

https://coderdojo.com/en/

I know John Carmack got his kids an Apple II and taught them BASIC.

Might look at things like Adafruit's Circuit Playground (BLE or Express). They can be used with MakeCode, which is similar to Scratch, and have fun I/Os.

There's a bunch of great programming games available on Steam. A lot of them start pretty basic, so might be useful for teaching the basics and driving interest.