How old are the boys? Assuming they are youngish I'll suggest Clifton gardens. Nice place for Mum too if she wants to relax. Playground, Picnic tables,Toilets, showers lol beach and a real nice Jetty for fishing.
http://www.hellosydneykids.com.au/clifton-gardens-mosman-sydneys-best-beaches-for-kids/
Bear