We stayed there an my kids love it. They want to know when we are going back.

They were 6 and 4 at the time.
They have the penguin walk and a free nighttime show where they will let children sit in a special section up front. (I think they paraded in if you signed them up ahead of time.)
There are paid activities as well at HHV.
My kids also liked the Atlantis Submarine. I was watching my baby niece at the time. So dad took them. They climbed Diamondhead. They loved the train at Dole Pineapple plantation.
And they did enjoy Pearl Harbor and were well behaved at the memorial. We also did the USS Missouri which does have an admission fee, but I think boys would especially love that. My kids enjoyed it. There was a kid activity that involved rubbings--but it has been a few years, I don't quite remember it exactly.
Since we were there for my 4yo ON her birthday (She was 3 when we went and 4 when we left), we went to Chuck E Cheese for her birthday. Not a must do, but it is a place to take the kids.
Polynesian Cultural Center had kid's activities and then we had a coupon from their goody bag for a free happy meal at the McD's right outside of it. We did not stay for the luau.
One difficulty we had, the time change. We flew in from the eastcoast. having gone on an earlier trip with the 4yo when she was 23 months, I planned this trip to arrive in time so that we would get to the hotel before sunset. This way, the kids would stay awake on the ride from the airport to the hotel. (We had a direct flight from Atlanta, very nice!).
That free show that the kids can do the parade...I think we may have done that on the first night, maybe the second. My 6 year old fell asleep during the show.
One note--the "driveway/street" out in front of HHV with all the shops, can get a little congested at times. So just be careful with the kiddo's.
On our last day, we did a buffet at a hotel that had an aquarium in the dining room--but I have no idea what it was called.
Many of the touristy things can work with the little ones.
We did have a rental car to navigate the island.
For the luau, we went to Paradise Cove. We took a tour bus to it.
We also hung out at the swimming pool.

And for the first 4 days, breakfast was bright and early at 5am when the first breakfast place opened.
