It's opposite The Hole in the Wall on Redcliffe Wharf. It was packed with kids all playing in the sand.
You can hire deckchairs .. there's food, a bar (not unreasonably priced), even a man selling paper-back books.
I left when ds threw sand in my face for the tenth time, but I could have happily idled away the afternoon there.
Take a look next time you're in town.