There aren't many true beaches in the immediate Bay Area. Santa Cruz is obviously the best one but if you don't want to travel so far and you're more into the 'beach experience' than swimming then Crab Cove is the place for you. It's easy for kids to walk along the water, enjoy the views and pick up shells or rocks at the edge of the water. There are also some good exhibits at the visitor's center and some nice hiking trails.
This is also a good place for kite-flying!
Best of all, this is somewhat off the beatn path so it is not nearly as crowded as, say, Ocean Beach in the City.