Take a look at Monkey; it's $120, but they can be very flexible if you contact them to explain the situation.

However, the demo provides an unlimited (non-commercial) HTML5 target for free, so coupled with a decent modern browser, you could teach them all for nothing. (The same code can later be compiled for XNA/Android, etc, in the full version.)

Commodore's Amiga User Interface Style Guide was really good in its day, and though of course Amiga-specific, it pushed hard for consistency and clarity of communication to the user in the little details such as when to 'gray out' inapplicable menu items, buttons, etc:

Amazon link

I think it's still worth a read if you can get it cheap. Amiga applications benefitted greatly from the Style Guide, even if criticism of a particular developer's app might have come only from a third party who'd read the book. Developers appreciated the way other "Style Guide-compliant" apps worked so nicely together and adapted their programs to suit, resulting in the majority of applications becoming consistently laid out, and therefore very intuitive, for the end user.