The victim was in the back seat of a Mazda that drove into the back of another car sitting in a line of stationary traffic.
He died at the scene.
His mother and younger sister were taken to Hawke's Bay Hospital. The girl has serious injuries.
The occupants of the car that was hit were also taken to hospital with various injuries. Both families are believed to be from Hawke's Bay.
A hospital spokeswoman said staff were treating a 5-year-old girl and a woman in her 30s, both with serious injuries.
Another woman in her 20s and her two children were in a stable condition.
Senior Constable Cory Ubels of the Serious Crash Unit said a woman was driving towards the airport, approaching a queue of traffic stopped at Pandora Bridge where roadworks were being done.
Temporary traffic signals were operating, allowing traffic to cross the bridge on a single lane.
The car crashed into the back of a 300-metre queue of traffic, hitting the car in front "pretty hard".
Mr Ubels said the driver of the car in front of the one hit had heard the approaching car and moved out of the queue and onto the side of the road.
That meant it was not also hit from behind.
Mr Ubels said the crash could have been a lot worse.
There were signs alerting drivers to the roadworks but Mr Ubels said the police were investigating if it was appropriate.
Roadworks at the site have been suspended temporarily out of respect for the families involved in the crash.
The Napier City Council did not respond to requests for comment.
New Zealand Transport Agency Napier highway manager Chuck Dowdell said their thoughts were with the family of the boy.
"We expect the work on State Highway 2B will be completed by the end of next week," he said.
He added that there had been no other reported crashes in the area since the roadworks started.
"All roadworks sites must comply with minimum standards for traffic management."
He said the site on State Highway 2B had an approved and compliant traffic management plan which included appropriate signs.
"Sites are continually monitored to ensure that compliance with traffic management is maintained," Mr Dowdell said.
If the need for additional traffic management, including signs, was identified while roadworks were being done, those would be put in place.