Just a little word of caution, jools. The big fireroad bridleways are actually some of the most dangerous places to ride with novices. Up yes, but not down. Speed is too high and things can and do happen that you don't expect. Choose twisty woodsy singletrack which is fun and feels fast without actually being fast. You might find that both Yog and BKB are too much for some – don't forget that what is dull for you will probably be great fun for them. Also, although they are all fit be careful as you get to end of the ride – don't save the hardest til last.
As for the core skills, well I can't give it all away! But head up and heels down are 100% essential all the time (no sitting if you're freewheeling). Plus braking is never just pulling the brakes – being stood up, head up, heels down, rocking back as you brake is probably the best thing to teach them, followed by just rolling over a log or other bump, to demonstrate that the bike will deal with the small stuff an not to react to it.