Plan on 7 days on the Big Island if you can, and 7 to 10 if it’s the only island on your trip.
Big Island
Big Island Itinerary With Kids: When to Split Your Stay and When Not To
For a week on the Big Island with kids, I’d split your stay: about 4 nights on the Kona side and 3 nights on the Hilo side, and see Hawaii Volcanoes National Park from the Hilo side.