There are calm waters for your children and adults alike. It is very crowded though with limited parking so plan accordingly.
Wayfarersbard
I don't know about parking as we lived 4 blocks away and would walk from the house, but always a favorite destination for me and my two young children! Pleasant breezes and convenient to the Kalapawai market for snacks and drinks. Watching the kite surfing was always interesting and exploring the rocks towards Lanikai was a great pastime for the kids.
23annae
This place is great for swimming with kids, body boarding, or just lounging on the beach. It's great that they have bathrooms and showers for rinsing off sand. The views are great from here as well. Lots of parking, too!