Two men aged 35 and 50, and a woman, 41, died at the scene after their black VW Touran people carrier collided with a black VW Golf car in Sheffield on Friday evening, South Yorkshire Police said.
The little boy died in hospital.
A three-year-old girl has been taken to hospital with '˜life-threatening injuries', while one woman, 22, is in a critical condition and another woman is in a stable but serious condition.
Police were in pursuit of three men in the Golf which was driving towards Darnall along Main Road when it collided with the people carrier just after 8.50pm.
The trio, aged 17, 18 and 23, have been arrested and remain in custody after suffering minor injuries.
The Independent Office for Police Conduct (IOPC) has launched an investigation.
IOPC regional director Miranda Biddle said: '˜As soon as we were informed, we sent investigators to the scene and we will be carrying out an independent investigation to establish what actions South Yorkshire Police took prior to the crash.
'˜This is standard procedure when lives are lost following an incident involving the police.
'˜By attending the scene we have been able to view dash cam footage which confirms that although a pursuit did take place prior to the crash, there is no indication that the police vehicle collided with either of the vehicles involved.
'˜We have also obtained initial statements from the officers involved and will gather further evidence during the course of our investigation.'
Detective chief inspector Jude Ashmore, of South Yorkshire Police, said he was thinking of the victims' families following the '˜incredibly tragic accident'.
He said: '˜A high number of resources were deployed to the scene last night, with officers from all of the emergency services working through the night and into this morning to assist.
'˜Our priority now is to support the victims' families and those affected, and work hard to determine the exact circumstances around what happened.
'˜Main Road is expected to remain closed for most of the day as we continue to gather evidence and recover items from the scene.'
Anyone who has information or dash cam footage should contact the force via 101.