Robi said
it can be more dangerous than fun for them.
I'd say that is the single most important point. We can help ruin an opportunity, or create a life long "bug" by exposing them to potential problems to avoid, until experience or curiosity takes them to that next level.
Make sure we've pointed out basics. Explain the "no sail" zone (breifly, as to detract from it being a negative) and beyond that you go anywhere. Staying upwind of "home" and the work involved to get back from downwind. Sail with the person until they're ready to go it alone. Show them how to right the boat, show them where to "never" dump the boat (and explain why), and what to do if they do dump it.
Emphasize the basic safety items, and keep tabs on them, ask them how it's going, but let them also discover on their own and ask questions of you.
