On Kauai, head to LYDGATE PARK, at the mouth of the Wailea River on the islands Coconut Coast (east shore). A huge man-made swimming oval is protected from the breakers, and even has enough fish to make for good snorkeling. The lawn, playsets, and picnic pavilions of Kamalani Playground are next to the beach, as is the Hauola Refuge, a cultural site. A short walk away is the enormous Play Bridge, with several stories of stairs, ropes and ladders. Miles of open sand invites beachcombers. Also on Kauai: Poipu Beach Park and Salt Pond Beach Park, both with the goods for a family day.
On Oahu, let's go with KAILUA BEACH PARK, on the windward side. Kayaking to Flat Island, just offshore, adds a splash of adventure to this popular park, which also has acres of gardescaped picnic areas and white-powder sand. Snorkeling is good, but a little better at sexy Lanakai Beach, adjacent to Kailua. Also on Oahu: Many acres of lawn and huge trees draw the locals to Ala Moana Beach Park, just across the yacht harbor on the north end of Waikiki. Magic Island, a man-made lagoon, provides safe swimming, and many people like to lap swim along the protected shoreline of the park. A deluxe mega mall is just across the palmy boulevard. It's also hard to beat Haleiwa Ali'i Beach Park on the north shore, which has all the amenitites, plus a world-class surf scene.
On Maui, families flock to the KAMAOLE BEACH PARKS (picture below), three of them side-by-side in the family resort town of Kihei. Each park has very nice shaded picnic areas. The beaches are scoops of sand, punctuated by lava points that provide fish habitat for excellent snorkeling. Lots of take-out eat options are nearby, as is a coastal trail that continues south to the five beaches of the Wailea resorts. Also on Maui: Pu'unoa Beach (Baby Beach) in Lahaina is short on facilities, but the swimming is fabulous and offshore islands provide a hypnotic view. On the windward coast, near hippy-dippy Paia, is Baldwin Beach Park, which could very well be in the "top five." A sweet baby beach, long beach walks either way, and good boogie boarding attact a wide range of visitors.
The Big Island has a ton of very different parks, but a popularity contest would be won by HAPUNA BEACH STATE PARK, on the north end of the South Kohala Coast. Picnic pavilions line a path that drops through trees and gardesn to the island's largest sand beach. The best snorkeling is a walk (or short drive) at Wailea Bay (a.k.a. Beach 69) also a state beach. A coastal trail also reaches the lovely sands of Mauna Kea Beach, at one of Hawaii's premier resorts. Also on the Big Island: You can't beach Kahalu'u Bay in Kona for family snorkeling. On the Hilo side are three beach parks that would top the list, IF sunshine were more frequent there. Not far from town on the shores of Hilo Bay, are Carlsmith, Onekahakaha, and Richardson Ocean Park. All have protected swimming, beautiful backshores, and a great family beach scene.
Alright. We fudged and only name four as "best," along with lots of honorable mentions. You get to decide the fifth, and it may not even be mentioned above. Pick up a Trailblazer Travelbook and give it a read before visiting Hawaii. There is a lot of beach exploring waiting to be had.