DH and I were in HH in January. We didn't do the Dolphin Cruise, a little to cool. The one really nice day we had we rode our bikes on the beach and went shell hunting.
We met a family that did the Dolphin Cruse and they loved it. One of their daughters is into marine life (daughters were under 15). Said dolphins came right up to the side of the boat.
There was plenty to do. We did the Beach Nature Walk with Bryan and he showed us how to find shells and what to look for (hence the reason we were on our bikes the day after). Bryan was great with all the kids (highly recommend this tour).
There were campfires at night, art classes, unbirthday parties, magic and music & lore of the low country. Highly recommend anything with B'Lou Crabbe (very interesting man and also great with kids).
Have a great time -- there is plenty to do.