Police have charged a driver after two children died in a car crash in Sydney's south.

Emergency services were called to The Grand Parade at Monterey, after a Subaru Impreza sedan left the road and crashed into a tree before 10pm on Friday.

Police will allege the driver lost control of the southbound vehicle and crossed to the incorrect side of the road before the impact.

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Two boys, aged 10 and nine who police said were the driver's nephews, were taken to Sydney Children's Hospital at Randwick where they died.

A nine-year-old girl, the driver's daughter, was taken to Sydney Children's Hospital with minor injuries.

The 33-year-old driver was taken to St George Hospital under police guard.

Police visited him at about 4pm and charged the man with two counts of dangerous driving causing death, and one count of causing bodily harm by misconduct.

The Beverly Hills man will appear via bedside hearing on Sunday.