All of the Hawaiian islands have their charms, but personally, I'd recommend Maui. It is like two separate islands. The west side is relatively dry, and there is a lot to see and do there. The east side of the island receives much more rain, and it is therefore very lush and tropical (like Kuaui).
On the west side, the snorkeling is terrific. Take a bike ride down the volcano (Haleakala). Lahaina is a fun town. A lot of the hotels have GREAT pools that you and your family would enjoy (for example, the Hyatt or the Marriott in Kaanapali). There's a marine preserve on the northwest side of the island that has great snorkeling (starts with an "H"--has a lot of vowels after it). You can tour a sugar cane facility, or a pineapple plantation. There's world class golf, if that's your pleasure. Macademia nut pancakes... umbrella drinks...
To the east: Take the road to Hana--just don't tow anything larger than a Bambi
The Hotel Hana Maui is VERY nice. There's even a State Park campground outside of Hana, with a black sand beach. The seven sacred pools are something to see, too. Lindberg's grave is just beyond that (Like us, he was into silver things held together by rivets).
After having written all that, I guess the one thing you must see while you're in Hawaii is the active volcano on the "Big Island" (Hawaii). Not too many places in the world where you can see something like that. If you're flying in and out thru Honalulu, spend some time at Pearl Harbor and the Arizona Memorial.
In short, you will have a great time. Maybe you can break up the trip by a stop at the "4th Annual Rocky Mtn. VAC Rally." Have fun!