Angela - You might want to check out something with NCL and I think it's about 7 days long. There is an island a day, plus an overnight in Maui on some itineraries.
There are tons of things to do in Hawai'i, but your kids might be at the age where they'd be happier in a hotel in Honolulu. Six might be a bit young for some of the Nature experiences, like Waimea Canyon, Haleakala, Volcano National Park. The islands are, however, very kid friendly - Hawaiians love kids!
Princess also offers a cruise, but it's much longer and has lots of sea days, which might be hard on your kids as well. I would suggest definitely looking at one of NCL's island cruises instead.
My first suggestion would be to find a local TA and see what they suggest.