The Lagoons at Ko Olina is our favorite beach to go to to swim and play. The entire thing is man-made, but don't let that bother you. This is an awesome beach, especially for kids. I know a 3-year-old who got knocked down by a big wave her first trip to the beach and has been leary of the ocean since then. But because the rock barriers protect this beach from the open ocean, it's almost like swimming in a pool, and even she will get in the water here. Plus, as I recently learned, there's some decent snorkeling along the rocks toward the back of the lagoon.
Here's what I know about the lagoons: there are 4 (pretty much)identical lagoons in a row. They are all beautiful, but we generally opt for lagoon #4 because it has the most parking. You can plop your beach chair down in the sand near the water, or claim a shady spot in the grassy area surrounding the lagoon. The only problem is the parking. There's not much, so it's best to arrive early (by 9 a.m. or so), especially on a weekend.
This place gets two thumbs up from me. It's all a family could want from a day at the beach (unless, of course, you are looking to surf or boogie board).