Hello,
Here is my personal thoughts on this:
I would get an Arduino anyway. It's not too expensive anyway and your kid can quickly and happily go on his own way. There are a lot of nice peripherals (OK, shields), examples, tutorials and so on. Results will be quick and plentiful. From an experimentation perspective, it's wonderful: you'll find shields for everything you'll need (finances and affinities allowing), writing code is really easy, etc
Then, I would try to learn a new processor together with the kid:
- What do we need to play with the new CPU? Yeah, Arduino is nice 'coz we have everything already provided in a board. What do we need to roll our own solution?:
- What the processor needs: supply, clock, etc. What are those and how we can built them?
- How we can actually access the CPU. What's a programmer?
- Hey, we have this debugging facility. How we can actually use it? What it is good for?
- OK, everything seems to work, but how we can actually instruct the CPU to do what we want? What's assembler and why I cannot use C
- OK, got it; I can use C, but this strange free C compiler from Microchip pretends that it's not able to generate optimized code? What's this story about?
- Oh, on the Arduino, we had all these nice shields and the code to play with them. Can we use them on the new processor. How we can proceed?
- OK, we've spent all this time and effort to reach the same solution as Arduino. Do we are happy about it?
- But wait, since we have all this experience, we can now understand that the Arduino platform is really just a CPU with a rather strange reset procedure. Can we take a new CPU, just the CPU and build an Arduino-like by ourselves?
- Yay, we can do it!
And so on.