We really enjoyed an afternoon at Hanelei beach park. My 4-year old got excited about surfing watching a daddy paddle-surf his little girl on his surfboard. our preschooler loved running through the water at the beach because it slowly rolls nearly 100 feet up the beach in places, the beach is so gently sloped.
this is a great surfing beach especially in the winter when the swells will find some spot to break & make a good line.
The pier is fun to walk down on.
Note that the water is murky so not ideal for diving or snorkeling.

Another beach that our kids loved, it has a gently sloping, wide beach and very gentle waves at shorebreak. this was in December. Lots of room to park a chair or towels. There are decent waves offshore for surfers, kayakers and paddleboarders to enjoy, they're easy to paddle out to, and a couple of rental shops for gear. You can reach it by walking down from the entrance to Duke's or I guess through the Marriott.

We wish we had found this beach earlier during our stay. Salt Pond has a gentle lagoon for adults to swim around in, and kids to play in. There are some spots where the reef creates a little pool right at shore so the really little tykes can wander in the "ocean" without getting pushed around by waves. There are lots of picnic benches.

We stayed next door at Marriott's Waiohai resort. Poipu State Beach Park is great. There's a playground and plenty of beach to sit at. You can also sit on the beach in front of Marriott; BTW, it's much less crowded.
Unlike other south shore & west shore beaches, the reef really calms the water so kids can play & swim. The beach is wide enough to provide some steep slope for older kids who want to do the boogyboarding stuff and other spots where the beach is less sloped and waves gently laps up on shore for less adventurous ones. Our 4 year old who is not a big water fan got the courage to run & in out of the water on our last few days. The beach extends slightly up to a tidepool, but unless you have surf shoes, it's tough to walk around and anyway there is not much to see.
The snorkeling apparently is good, i unfortunately had a broken arm so could not verify that!

BTW website is http://www.vizcayamuseum.org/
We visited Vizcaya museum a few years ago before the baby. It has something for everyone. Colonial architecture, art, fun gardens to wander around and some amazing views along the waterfront. Some of the maze gardens will probably be a lot of fun for the 5-14 crowd too.