I sail with my seven year old daughter and 5 year old son all the time. They love it. My son is already talking about singlehanding the 5.2 . I'm not quite ready for that and he needs about a hundred more pounds before he'd be ready. All you can do is show them the fun side of it, if they get the competative spirit, good for them. If they want to kick around the bay and occasionally fly a hull to show off, even better.
Rob V.
Panama City
Nacra 5.2