Feet should be approximately shoulder width apart with your feet slightly ducked (basically how you normally stand in an athletic pose).
Have you checked out
http://wakeworld.com/news/feature/fr...powerturn.html ? some good pickup info if you're new.
When you're trying if the boys are falling forwards they're just standing up too fast. I always tell people to stay in a ball/tucked until the boat is more up to speed. Don't hammer it too hard off the start, nice controlled gentle starts are helpful.
If the rope is getting ripped out of their hands advise them to keep their arms out straight, stay tucked, and not to push with their legs.
The idea of getting up on a wakeboard is no different than helping someone sitting off the floor, the person extends a straight arm to you, you pull, they roll onto their feet, and only when their feet are underneath them and their bum off the ground do they stand up.
Good luck and have fun!