if they have never been on a bike before your best bet would to have them ride in an open field and once they get the controls down you can try some trails as long as there easy.
i suggest that all new skills be learned in an area with easy access for emergency equipment. pushing a broken bike 5-miles is one thing but pushing a bike when you are broken is all together different!!!
I dunno, perhaps you should ask the guys over in the "What Bike Do I Get... ? " forum. But before asking, you should read teh very first sticky there which will make them a lot nicer to you. It is titled "<< Sticky! >> Hints to help get advice on bikes"