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.