Congwong Beach is a relatively small beach but that's one of its best features; it doesn't draw big crowds and is great for families who just want an accessible stretch of sand where they can get their toes wet.
The beach is located in La Perouse, off Anzac Parade, and can be accessed from the car park for Bare Island (in fact, you can even see Bare Island and its historic fort from the beach). It's a relatively easy, short walk down to the sand, but little kids might prefer to be carried on the way up, as it's a little steep in sections.
Part of Botany Bay, the beach is quite sheltered and doesn't have any surf, so you won't be competing with anyone on boards. It just offers a calm place to swim or even go snorkeling.
Despite being in quite an industrial part of Sydney, it's pretty beautiful too. At the back of the beach is Kamay Botany Bay National Park and from your spot on the sand you look over Congwong Bay to the headlands of Botany Bay.
At the end of the beach there is a path that will take you to Little Congwong Beach, but it's best that you don't follow it if you're here with kids (clothing is considered optional over there).