You are going to have a great time! My kids were 7 and 8 the first time we went to HHI. Some of the highlights for them:
Crabbing!-get some chicken (necks or wings) from Publix or Piggly Wiggly.
Banana Peels at the beach-kind of like a recumbent bike for the beach.
Campfire
Kid's night out-they had pizza and games and went to a movie. My husband and I went to a wine tasting.
Daufuskie Island-we went on a tour (just us on the boat) and got to get out and explore a completely deserted part of the island. Saw lots of dolphins too. We have also done shark fishing which is really fun.
Meeting Shadow
Kids' activities-crafts and games, etc.
The BEACH!
The pool slide
We have gone down to the Salty Dog and Sea Pines and now we don't. It is so crowded and the kids just aren't interested, even as young adults. I go out the first day we are there and get a bunch of easy to throw together meals from Fresh Market and Publix. We would rather be at the beach, the pool, crabbing, in hammocks, playing games on the porch, or riding bikes than waiting for a table. At the peak, even getting a pizza delivered can be a huge wait. We do go out to eat when we are at HHI during off-peak times. There are some nice places but, for us, nothing worth waiting (or as my kids call it, wasting our vacation) a long time for.
Have a great time!