A great introduction to the topic, thank you. I'm curious: is there a follow-up planned?
Things I liked were that you related the process of the design to solving a "real world" issue (although this did drop off, which I thought was a shame). You also skipped some of the more complex issues (natural versus surrogate keys springs to mind) which I personally thought was a good choice: it's far more important to get the concept of keys in place first.
I did find the whole Alice+Bob narration style a little trite... however, that style may work well for the intended audience, which I am well aware is not myself!
All-in-all, I felt it was an excellent introduction, and that you pitched it at exactly the right level; if anything, I felt you introduced a few rules very early that I would have put off for later, so kudos for you there!
Now, about that follow-up...