I'll have to check with the management at Extreme Windsurfing, but my guess would be that we can work with a couple of small kids, or you can enroll them in the regualr Extreme Windsurfing School.
How old are your grand children and what do they weigh.
That might be the determining factor.
If they are younger than about 6 yrs old or 70 lbs. I can take them out for a little ride to see how it all feels and probably have them sailing (with a little help).
If they are older, then perhaps they can sign up for the regular Extreme WS lesson program.
Hope this helps,
P.S. I'll try to give you a better answer tomorrow nite.