Any beach in Hawaii can be perfectly safe and fun one day, and then due to various weather conditions, morph into being not so safe, or even lethally dangerous, the next. Keiki (baby) beaches, as they are known locally, are an exception. These are where on most any day the kids can wade about at the shore or swim with parent's in nice calm coves.
Some keiki (KAY-key) beaches are naturally occuring pools, protected by a near-shore reef or rock formation, while others are man-made, or man-enchanced at least, where boulders have been placed to protect the shore. Here are a only few beach swims where waves and current are not an issue:
On KAUAI, Lydgate Park (pictured) is a crowd-pleaser with a shallow lagoon, beach and multi-tiered wooden playground on site. In nearby Kapa'a, Fuji Beach, is more of a local's scene. A long, narrow pool with sandy bottom is right beside palm trees in a sleepy neighborhood. On the west side, sunny Salt Pond Beach Park, will keep you coming back. Palms, lawns, big crescent beach, with little pools to each side,
On OAHU, Magic Island at Ala Moana Beach Park is the mother of all baby beaches. Its big oval pool abuts many acres of tree shaded picnic tables and lawns, where locals hang out on weekends, rather than nearby Waikiki. For a more mellow scene, go just around Diamond Head from Waikiki to Mothers' Beach in ritzy Kahala. On the north shore, take a ten-minute walk up the sand from Turtle Bay Resort to the tiny jewel of the Kaihalulu Keiki Pool.
MAUI has one of the best: Baby Baldwin Beach near Paia. A natural swimming beach is backed by a bleacher of shaded sand, with views of the West Maui Mountains. Nearby, is Spreckelsville Keiki Beach, known more to locals. You can expect privacy on week days. Across the island in Lahaina, you'll find sweet Baby Beach, with killer views of Molokai and Lanai.
Near Kona on the BIG ISLAND is Airport Baby Beach, a big sandy opening in a reef of smooth lava. Just north, Kekaha Kai State Park has a fantasy-island natural baby beach, but it takes a bumpy drive and mile-plus walk to reach. The most underrated kids' beaches in Hawaii are on the outer rim of the big bay from Hilo. Check out Onekahakaha, Carlsmith, and Richardson beach parks.
Trailblazer guides for each island have details on each these safe-swimming spots, as well as about ten times more. Hawaii has many nooks and crannies.