Got kids? Need to relax but keep an eye on them? Get on the Manly Ferry - so easy to use, and great fun as well as a super view of Sydney. When you get to Manly get off and turn left and keep walking until you can go no further on the beach and you'll be at a cove called Shelly Beach. This little horseshoe is for parents and kids - the water is cyrstal clear and about 8"-12" deep for a good 100 yards, so ideal for little ones with no menace, unlike the big surf furhter along.
Then, when you are all famished, wander 25 yards to 'The Kiosk' the misleadingly named and wonderful restaurant on the front there, and eat your fill of Morton Bay Bugs, Sushi, etc, etc for buttons. Great easy day. The other thing you could do is peel off to Manly market and buy a tray of mangos to eat on the beach - it will add half an hour to the journey out there though! Again, costs buttons as do most things out there.
Ferry leaves from Sydney harbour every hour.
Is this your business?
To get to Shelley Beach, you get off the ferry, go straight ahead along the shop and restaurant-lined Corso until you reach the ocean beach (about 200yds) then turn right and follow the seaside path to Shelley Beach. And I think you will find the water a lot deeper than 12" after 100yds. But generally sheltered, north facing, and good for kids.
The Kiosk is good but not cheap.