Duke Kahanamoku Lagoon in Waikiki

If you are looking for calm water for your kids to play in, consider checking out the Duke Kahanamoku Lagoon in Waikiki.  The lagoon was renovated several years ago and is now a giant saltwater swimming pool.  It is surrounded by beach on all sides but the water is circulated and kept clean and the beaches are also well taken care of.  On several sides of the lagoon there are nice grassy areas behind the beach that even have some shady spots from the trees planted there.

The lagoon is located next to the Hilton Hawaiian Village at the eastern end of Waikiki near the Ala Wai Boat harbor.  It is considered to be a part of the grounds of the Hilton but is open to the public.

The edges of the lagoon are extremely shallow and there are no waves here, so it's a great place for children to run and play and enjoy the water.

In addition to wading and swimming, the lagoon is also a popular place to try out kayaking or stand up paddle boarding.  These and other watercraft can be rented from a concession stand at the corner of the lagoon nearest the Hilton.

The only thing I don't like about the lagoon is the sand.  It feels dusty and a bit artificial to me and if you want to build sand castles you'll probably want to head somewhere else.  My kids have never complained about the sand so don't let that one complaint of mine keep you from visiting this spot.

Do it yourself on Oahu
Location:  The Duke Kahanamoku Lagoon is located adjacent to the Hilton Hawaiian Village on the Eastern edge (away from Diamond Head) of Waikiki.
Directions:  Get on Ala Moana Boulevard.  Between Ala Moana Mall and the Hilton Hawaiian Village you will turn Makai (towards the ocean) on Hobron Lane.  Then take a left on Holomoana Street and drive until you find a parking spot.
Parking:  After you turn onto Holomoana Street there is a paid parking lot on your right.  There is also free parking along the lagoon on the left and out at the end of the road on the oceanside of the lagoon.  I always check the free parking lot first but if you do end up in the paid lot the rates are very reasonable.
Facilities:  Drinking fountains and I think a water spigot for rinsing off your feet are next to the lagoon.  Can't remember if there's an outdoor shower or not.  There are no restrooms adjacent to the lagoon but the Hilton has restrooms you can use that are just a short walk away.  There's also a snack bar on the lower floor of the Rainbow Tower facing the sidewalk.
More info:  Here's a view of the lagoon from a little different angle.