First trip with kids for us was when our daughter was 3y3m. Great comments so far. I'll re-iterate bringing a good, well-fitting PFD that the child likes to wear. Ask for netting for the lifelines.

Also, the two things that made the time go by best for our child:
- Bring a pack of "bathtub crayons"... these are markers designed to make bathtime more fun by letting your kid draw all over the tub and tile. Rinses right off. Encourage them to go to town in the cockpit defacing that half-a-million-buck yacht! Cockpit table and bulkhead were always colorful and interesting on that trip!
- Go ashore to the beach, but bring some of the ship's mixing bowls, pots, spoons, etc. My daughter liked to mix bathes of sand meatballs and some other culinary delicacies.

Have fun!