Police were told about 4pm a group of youths were throwing objects at passing vehicles on the Northwestern Motorway, a police spokeswoman said.
"One of the objects allegedly went through a windscreen of a vehicle. There were no reports of injuries."
Police spoke to the youths and one person was taken into custody, she said.
"The person has been referred to youth aid and inquiries are ongoing."
The man who contacted the Herald estimated the toddler was aged about 2.
The windscreen of the family's car, a white station wagon travelling westbound, was smashed on the passenger's side.
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Four teenage boys were at the scene and six police cars arrived after emergency services were alerted, the man said.
The incident was "bloody horrible" and he kept thinking about the death of a man killed when an 8kg block of concrete was deliberately dropped from the Princes St overbridge on Auckland's Southern Motorway.
Taupō 20-year-old Christopher Currie, a King Country under-20 rugby team prop travelling to a game, died instantly when the block went through the windscreen of his car in 2005.
Three passengers were hurt when the car subsequently crashed and a 14-year-old boy, Ngatai Rewiti, was later found guilty of manslaughter over Currie's death, and sentenced to four years' jail.
He hoped the person responsible for today's incident faced appropriate consequences, the man who contacted the Herald said.
"We're lucky a little girl didn't get killed on the motorway tonight."
