Ideally something that would accompany "Code" book by Petzold?
All DIY kits I see on the market are already on top of a built computer (raspberry pi or whatever).
I see much more educational value in building CPU from scratch than just plugging peripherals in and out. I hope I missed something.
Understanding a working electronic CPU builds on many, many abstract concepts that need foundational knowledge established first. If they're not already doing pre-algebra, they're simply not equipped mathematically to understand the value or purpose of the exercise and you'll just need to make it fun instead. For any kid under the age of 10 I would just get them https://spintronics.com/ where they can eventually build a 1-bit adder mechanically: https://community.spintronics.com/t/1-bit-half-adder/310. Spintronics lets kids (or adults...) build intuitive understanding of electrical concepts with a set of puzzles or free play.
Beyond that, electricity might as well be magic to even huge portions of the adult population. Putting it to work to do computation even more so. If you have an especially interested and capable kid, start with basic electric circuits and build their skills up to using diodes. Then work your way up to using transistors to build an XOR gate. Then combine XOR gates to build other logical operations. Then you can build the most rudimentary of processors.
If that sounds plausible, then you could follow videos like this one: https://www.youtube.com/watch?v=_eo8l7HP-9U and build a 4-bit computer mostly out of LEDs, transistors, wiring and breadboards, but it's not like it accepts keyboard input or has graphics or anything.
In terms of DIY kits, something like https://www.qsoshack.com/make-a-new-zx-spectrum-from-a-kit/ is about the most basic form I think you might find that results in what is recognizably "a computer" by common understanding of the term, but the leap from an XOR gate to a circuit board filled with chips is still a big one.
> Refine the age of "kid", or at least where their abilities already are because this would be a big endeavor for most elementary school age kids.
"kid" was a bad reference point, Petzold's "COde" book is a better text equivalent of what I am looking for
The happy medium is probably using both, together. Raspberry Pis and Arduino boards all have documented GPIO you can use to leverage the power/simplicity of a modern CPU with the tactility and practicality of a physical circuit. If you're purely interested in educational value, building a CPU from scratch might not be a good idea for a young audience.
It depends on the audience.