How serendipitous! I recently blogged
about Cebes. If he comes up often in Athenaze, you might find some of the reference I link to useful.
The problem with the word δαιμόνιον is etymological. Because it's the ultimate origin of the English word "demon" — which is pretty universally understood negatively — using a transliteration from the Greek, "daemon" is likely to give a misleading impression to someone unexperienced with Greek. If you're writing for an audience in the know, then using "daemon" is less likely to mislead.
So, I assume the author of the workbook is used to writing for people who will have in mind the non-English senses of "daemon."
One often runs into these problems with special vocabulary when dealing with ancient philosophy.
As a random note, in the world of the Unix operating system (including Linux, which more people are likely to have heard of), a daemon is a special sort of background program that does work for the operating system and the users. At work, I get to talk about killing daemons.