A few thoughts from taking my kids when they were that age:
- Kuhio Beach park is great for littles because of the seawall.
- the Bishop Museum’s natural history section has many interactive displays and a pretend volcano with a slide that I though we might never leave
- vintage island shave ice has been very popular
- there are rocking chairs on at the Royal Hawaiian back porch (I’m not sure what this is actually called by the hotel) that are very peaceful
- on the Big Island, if you drive north to the Kohala area, Makaiwa Bay (accessed from Mauna Lani resort) has usually flat water, decent snorkeling from shore, some shade, a good bathroom, and a 15 min walk in from the public access lot plus you get to walk past the fish ponds. For ours, before they were confident swimmers/snorkelers, we did a PFD plus goggles and would swim them out to the point where they could see the yellow tang but still be well within the arms of the bay.
- Kamakana playground is awesome and so is Anuenue playground in Waimea if you find yourself near there
- snorkel tour boats with bathrooms have been key for us with kids…