We have been here in Oahu for 6 days and have checked out a ton of beaches.
So far, we feel the Kailua Beach / Lanikai is the best beach.

It has very clear water, lots of soft sand, and lots of shade trees in case you need them.
The mountains are right behind you and you feel like you are on the beach in Jurassic park without the dinosaurs, of course.

The water is pretty calm and super clear.  Snorkeling was great here.  Not a ton of sea life, but very clear water.

Enjoying the soft sand and clear water.

The water really is that blue / green.

From the water, you can see a forest right on the beach with beautiful mountains right behind the trees.
You have lots of shade if you get tired of the sun.Lots of restrooms on site with showers to wash off.

We felt it was a beach just for kids.  The three of us were surrounded by lots of other kids.
Football and bbqs were going on all over the beach. We will definitely visit this one again before we have to leave Oahu.