I like the vat of sunscreen comment.
Here's a few more.
First of all, learn to stop the boat quickly, heave to, and drift/sail backwards. It's often safer and faster that trying to do panicked figure 8's. Have a throw bag ready. You and the "crew" can practice with it on the beach.
For the long term, teach them to respect the BOOM and not get tangled in lines.. Short term, keep them warm (rash guards and form fitting life vests, hats, wind breakers and surf sox). And watered/fed, w fruit and trail mix at least (I threw some M&Ms in my dry mix and kept a preschooler occupied all day).
Plan your family trips around interesting sights or destinations, rather than your racing course.
Games that focus on your environment are great (which way is the wind?, how's my-tell tales?, 20 questions, punch-buggy style games like spotting marine life, birds and other boats).
Singing along the way is fantastic, and helps avoid boredom and seasickness.
Oh yeah, avoid seasickness at all costs,.. and don't forget the camera!