We visited HHI last September. Our sons were 26 months and 6 months old and we had a GREAT time!
Our kids enjoyed the pool and the beach. They also enjoyed the story time with cookies/milk and the campfire/s'mores. (They weren't really old enough to participate in the songs at the campfire, but had fun dancing along... even the 6 month old was smiling from ear to ear.) We also rented bikes and the kids enjoyed riding along.
We also all enjoyed a dolphin cruise (we booked this outside of the resort as the resort tours were full and/or cancelled due to weather). Our oldest son (2 years old at the time) LOVED the cruise. The baby did well, too.
We also took the island tour offered by the resort. While the tour was wonderful, I don't recommend it with little ones. We enjoyed it but, only because we had both the kids' grandmothers with us. We would not have attempted this tour if we didn't have extra help to entertain the little ones.
I hope you enjoy your trip as much as we enjoyed ours!