I personally love this macropad tutorial and it's how I got into doing circuit board design in highschool: https://hackpad.hackclub.com/. Before doing this, it's really helpful if they have experience messing around with breadboards, so getting them a kit can help them into a soft transition too!

I like to suggest making a macropad, then a keyboard, then going into a devboard, and then starting to make your own projects. But for kids that aren't quite in highschool yet, it can feel quite intimidating and the learning curve is moderately high, so getting them playing with breadboards first might be the best option ;)

Hack Club will also give you a grant to cover the entire cost if your kids are over 13 through their programs like stasis, fallout, or forge (you can check their site for more info)

I do workshops with kids occasionally. Last week, 4 13 year old boys. In this case I did breadboarding with them first and then showed them the transfer to fritzing -> breadboard -> schematic -> pcb. https://fritzing.org/ If you're looking for stuff they might find fun, logic noize (for instance https://hackaday.com/2015/03/09/logic-noise-sawing-away-with... ) has a bunch of fun cmos audio tutorials with great videos. Personally, I build audio toys, both analog and digital (mostly pi pico2) and still mostly use fritzing for the breadboard education element, but kicad if I need smd positioning and the like.

for an introduction to the CAD software, Kicad, which this project uses, I really liked "Shine on you crazy Kicad": https://www.youtube.com/watch?v=eMW9ohCbcik&t=27s It takes you from basics to buying a PCB from the manufacturer.

Breadboards[1] with breakout boards[2] (the Pi Pico in particular is incredible, having an unusually detailed and correct datasheet). MicroPython is probably a great way for gets to get started on the compute side of things. Once you have something working on a breadboard then you can "graduate" to PCB design (and there's nothing wrong with designing the PCB to take a breakout daughterboard) - possibly perfboard as an intermediate step. Use KiCAD - it's Blender-tier open source software. A high quality breadboard and wires are very important - I guess diagnosing a loose connection is a learning experience, but it would be an overall hindrance to learning if encountered frequently.

Have many spares of all components (including breakout boards) handy - they will probably summon the magic smoke a few times.

They will likely quickly need to learn how to read data sheets. You can often get away with copying the "typical application" and avoiding the real technical stuff.

Friendly reminder: those brains are still developing. Adequate exhaust is extremely important (even if you are using solder-free, flux fumes are varying levels of toxic). You want those fumes going out of the window, not via a filter into the same room. Any complaints about headaches are serious: you aren't extracting the fumes correctly. There are a few important things to know about soldering: "there is no such thing as too much flux," "solder likes to flow towards heat," and "heat the thing and apply the solder to the thing (not to the iron)" (to keep the technique at its most basic).

The often recommended Hakko FX-888D is just plain awful. The Pinecil is way better (yes, even though its a fraction of the price) or the TS100/TS101.

[1]: https://en.wikipedia.org/wiki/Breadboard [2]: https://www.raspberrypi.com/products/raspberry-pi-pico/

Strongly feel the logical progression is: (1) Wiring third party devices and modules to an existing MCU board and programming it (2) Making a PCB to plug modules and the MCU board in to (3) Making a PCB with integrated peripherals and only plugging the MCU board (4) Making making a whole board with everything.

The MCU is typically far more fiddly than the devices (eg. crystals, storage, buses with conditioning, power stages, etc.), so continuing to plug the MCU in to a PCB while integrating peripherals is a good. You really need to be able to read a datasheet to do a nontrivial board and that brings in quite a few elements of electronics which are nontrivial for kids to grok without hand-holding and a lot of explanation.

Pick an MCU with easy USB-C programming. RP2040 is a good modern option.