But the watchdog described the breach of the editors' code of practice as "slight" because the reporter soon realised his mistake and did not publish any of the material from the brief exchange.
The Post reporter had knocked at a number of houses in St Asaph in Denbighshire seeking information about a serious road accident which had left the victim critically ill in hospital.
The girl answered the door and confirmed where the accident had occurred and the name of the boy who had been hurt.
At this point, the reporter noticed the girl was wearing school uniform and immediately left.
The girl's father, Phil Adey, said she had been distressed by the reporter's questioning.
Clause six of the editors' code of practice says a child under 16 "must not be interviewed or photographed on issues involving their own or another child's welfare unless a custodial parent or similarly responsible adult consents".
In its ruling, the Press Complaints Commission said that although the exchange was brief it still counted as an interview.
"While the reporter should arguably have been able to deduce immediately, from her school shirt, that the girl may have known the victim and was under 16, he did withdraw after a brief exchange when this occurred to him," the commission said.
"It is clear that, with the benefit of hindsight, he should have taken greater care not to engage the girl in a conversation about the accident.
"But nothing from the interview was published – so there was no public impact on the complainant's daughter – and the commission was satisfied that the breach of the code was not serious."