Boy, 4, drowns after car rolls down boat ramp.

What should have been a fun day out has turned to tragedy when a four-year-old boy died after being trapped in a car that rolled into the water off a boat ramp.

Yesterday Bentley Hilton, 4, was out with his father and grandfather at Lower Portland caravan park on River Road Hawkesbury River, north-west of Sydney.

Officers said the ute, being driven by the boy’s grandfather, was reversing a boat into the river when the vehicle slid into the water.

Police pulled the boy's body from the water. Via 7 News.

The grandfather had gotten out of the vehicle in order to get the boat into the water.

"He got out of the car to get a small tin boat off the back of the ute," Inspector Ian Woodward said.

"A young four-year-old child was seated in the cabin of the vehicle.

"As the man got out to unstrap the tin boat, the ute rolled back into the Hawkesbury River and he was submerged.

The boy and his father and grandfather were at Lower Portland caravan park on River Road Hawkesbury River, north-west of Sydney.

"The man made frantic efforts to try and get the boy out of the vehicle but he was unsuccessful," Inspector Ian Woodward said.

"The child's family and a number of police officers went into the water to try and recover the vehicle and the child, but they were unsuccessful in their attempts," Inspector Woodward said.

He told media that Bentley’s family was on holiday in the area and are now devastated.

The ute was pulled from the water several hours later. Via 7 News.

It is understood that both the boy’s father and grandfather entered the water several times trying to save the trapped boy.

“There is a sharp drop-off so they were unable to get to the vehicle or the child,” Inspector Woodward said.

His mother, Lauren Hilton from Penrith told The Daily Telegraph it was “hard” to believe Bentley’s life had been “taken away so quickly”.

“He was always happy to see his mum, see his sister and see his family,” Ms Hilton, a mother of two, told The Daily Telegraph.

“He was a happy four-year-old. A bright little boy.”

She said she last saw her son on Mother’s Day but she had spoken to him on Saturday night.

“He was happy telling me about everything he was doing. We were laughing and giggling.”

Divers spent a number of hours extensively searching the waters surrounding Lower Portland caravan park on River Road before recovering the little boy's body from the river over four hours later.

Paramedics performed CPR on him but he was pronounced dead at the scene.

Police said the car moved with the current and divers eventually located the ute about 20m away from the end of the boat ramp yesterday afternoon.

Our thoughts are with Bentley's family.