Hawaii, and many more dozens of beaches that families will call their favorite. But if for some bizarre reason, only one park could be picked, Lydgate Park on Kauai's east shore (the coconut coast) would be at or near the top.
Start with historical fact: The first of the ancient Polynesian mariners, some two thousand years ago, sailed up the Hawaiian chain and chose to call this spot home, where the verdant Wailua River meets a long sandy beach. Cultural sites here are one of the attractions at Lydgate.
The main draw though is the large man-made swimming oval that is always safe for swimming and big enough for snorkeling, as a few fish get tossed by waves over the protective barrier. (After severe rainstorms, the swimming pool may become full of river-borne driftwood, the only exception to the safe-swimming rule.)
Facilities and showers are up to snuff, and a large lawn across the huge parking lot sports Kamalani Playground, where kids can take a break from sun and surf. A three-story play bridge, whose chutes and ladders can absorb an entire kindergarten class, is about a half-mile away, near Lydgates campground.
Beachcombers can walk about two miles along the strip of yellow sand to Kauai Beach, and kayaking families can try the placid waters of Wailua River (or take a ride on a paddle boat at Smiths Tropical Gardens to Fern Grotto).
Lydgate is set well off the highway near Kapa'a, and enjoys plenty of sunshine without being arid, like the south coast. A downside is that popularity means lots of people show up, but there is plenty of space and parking is not an issue. The scene is always family friendly, for locals and Mainlanders alike. Complete directions to the park and surrounding activities can be found in the Kauai Trailblazer guide.