I cannot recommend HHI enough - we stayed in August and we are going back in March - I have a soon to be 5 year old.
He talks about the resort so much at school and home that his teacher asked us if we owned another house in Hilton Head and that was his first trip!
The cast members are amazing, they have a massive amount of activities for kids that age both in the pool and outside the pool, both paid and unpaid. DVC discounts for all activities.
Our son did the slime activity, we made shirts together, did a painting activity together, we did a dolphin boat tour that left from the Disney Resort Boat dock, he participated in the pool games led by the CM's, we attended the story time and magic shows by B'Lou...there were so many activities we had to actually skip some.
We already have a list of things we need to do for our March trip.
There is a shuttle to the Disney Beach House - we never waited for it - you can call and they just come get you and it's probably a 3 minute drive and by the way, the shuttle itself and the person driving it, are also fun activities. My son wanted to just ride the shuttle back and forth with Mr. Stan or Mike the Pool Guy.
What time of year are you thinking of going?