On the Big Island I'd recommend
Hapuna Beach it has great big sandy beach. For
Black Sand Beaches you'll need to head south of Kona. I'd recommend you rent a car if you have the time. If the volcanos are active when you go I'm sure the kids would love to see an active volcano. Its about a two hour drive south of Kona in the
Hawaii Volcanoes National Park. If you have any more questions just ask.
Happy Travels,
Dan