You can't reasonably expect a child to understand hardware virtualization, can you? Much less how to use something like Docker. Maybe if Docker was perfect and there were no version or dependency incompatibilities, and the Docker service behaved predictably, and a bunch of other things. Hell, I would've bailed a few minutes into the build process as a child. I get bored waiting on it as an adult.
Also real-life hardware can be super interesting, even if you're not hacking at it! I found much joy just cracking open the family's old Windows machine and having a look. It's hard to quantify the effect something like that has, but it's got to be greater than zero.
However, for our daughter we ended up buying (1) a Micro:Bit, which is much easier for kids to tinker with, especially with MakeCode [1]. Our 5yo daughter loves to draw 5x5 emoji + write words, associate them with buttons, to upload them to the Micro:bit. It makes her really proud. (2) An Intel NUC, which is much faster and less constrained than a Raspberry Pi. (Obviously, it is more expensive, but she has one of the low-end Celeron NUCs.)
Edit: I write this with several generations of pi. sitting idle on a shelf. The most interesting project I used one for was Halloween automation of off the shelf decorations. Made up for the stagnant pi's sitting around and all the neighborhood agrees.
Why would anybody care that they're being bought and put in drawers or shelves? The RPi isn't in finite supply, and buying more helps the project overall even if nothing is done with it.
As for teaching your child docker being easier than flashing an SD card... you must have some bright kids, pal!