She died in hospital on Saturday night, Capital and Coast DHB spokesman Lindsay Davis said. Her mother was still in a serious condition in the intensive care unit.
Inspector Marty Parker, from central police communications, said the crash happened on SH1, near Poplar Ave, where the lanes merge from two to one.
Kapiti Coast police were investigating.
Senior Sergeant Stan Wagner of Wellington police said witnesses were being interviewed yesterday and the matter had been referred to the coroner.
Although it was too early to know the circumstances in the two crashes, Assistant Commissioner for Road Policing Dave Cliff said it was crucial children were restrained correctly.
Legislation changed in November, requiring all children up to the age of seven to be restrained in approved carseats.
"That was all for the reason that research says a child needs to fit an adult seatbelt correctly to be safe," Mr Cliff said.
The other message was around avoiding crashes, he said.
"So keeping speed down, not driving while drowsy, not driving after drinking alcohol, being attentive - those things become really important when you've got kids on board."