Coogee and Manly Beaches. Otherwise any of the beaches on the Central Coast (except for Terrigal) are also fantastic. Speaking of a near Sydney experience, The Entrance has a fantastic beach, with free pelican feeding sessions at 3pm sharp.
Balmoral Bech is great as it also has grass and, most importantly for kids, shade. Otherwise there is Clontarf, which has more of a bay than a beach but also grass shade and play equipment. My best outside Sydney experience is Narrawallee beac on the south coast, right next to Mollymook beach. When the tide goes out the massive rockpools there fill with water (and some harmless wildlife). Here's a picture of my son exploring one. :)