I even mentioned libc. There is more to an operating system than a kernel.
In fact, this was an important part of my early computer education: when I was 12 (1994) I bought a copy of Slackware Linux because it had a free compiler (even Turbo Pascal was expensive for a 12 year-old).
At the beginning I just typed some small C snippets that I could find in books in the library. Then I started reading shell scripts and found out that a lot of the system is controlled through shell scripts and started writing my own. Then I started reading through the source code of libraries to find out how things were implemented.
At some point I got a Minix book (I was probably 16) at a discount at a book store. I started comparing the theory I read there and tried to find out how e.g. the Linux VMM differed from Minix.
Don't underestimate how important it is for a kid to have access to such information. Only a small subset dive deeply into the system, but they will be the hackers of the future. But you have to awaken the interest by making easy to dive in.
They will be using some app/language running on the OS. They don't need access to the OS source to do that.
I think you underestimate how important access to the OS (in the widest sense) has been to the success of everyone from Google, Dropbox, and Facebook to the newest of startups.