Faster's post is right on.
My wife and I are taking our two girls, ages 9 and 7, on a four-day cruise along coastal South Carolina and Georgia on our C30 on Oct. 25-29. We're going to have them keep a log of what they did and saw -- sort of make them play "marine biologist" for a few days. We've put a lot of miles under the keel together, and we know that keeping them interested in what's going on can be a bit challenging at times. I'd also emphasize Faster's point about teaching them to sail. Take paper charts along, so that they can see where you're going. Get them at the helm.