Maui
The road to Hana is amazing but perhaps too much (boring) for the 6 year old? We did this drive on the second to last day of our trip. I changed out our rental car for a 4 door jeep. Was amazing with the roof tops off so you don't have to strain to see things. Kids loved the Jeep. I would plan to hit the road really early and spend a long lunch at the black sand beach. Lots of cool stuff and hikes there. With the Jeep we elected to continue all the way around the island rather than back track. The scenery changes from tropical to desert quickly. Very cool...err dry and hot
Big Island
I wrote up
THIS REPORT on BIG ISLAND a while ago. With your 6 year old I think the Hilton would be a solid hit. They have a man made lagoon for snorkeling that is stocked with turtles and fish. Max depth is maybe 8-10 feet. Protected from tides and stuff. Very approachable and fun for the kids. Lots to do there but note the walks are long and the train/boat service is very very slow. Dolphin encounter if that is something of interest. Bring your money - as you would expect the food/drink prices are nuts. We stocked up at Costco on snacks and drinks prior to check in.
Have fun.