A judge has thrown out the prosecution of a caregiver who poked a jagged broken pen into a boy's leg "to teach him a lesson", leaving him bruised but not badly injured.

The case in the Waitakere District Court appears to set a precedent that could let other parents escape conviction for what are technically minor assaults against their children under the controversial 2007 "smacking" law.

But Lorraine Cummins, the Henderson mother who poked the boy with the pen, still had to endure a night in a police cell and spend $3500 on legal fees before Judge Ian McHardy threw out the police case against her.

Family First director Bob McCoskrie, who cited the case in full-page advertisements on Tuesday as one of seven new examples where the new law was penalising good parents, said other parents had been convicted for similar minor assaults.

"So it could have gone either way," he said. "You still run the lottery of whether the judge thinks you are 'preventing' or 'correcting' [a child's behaviour]."

The Henderson case involved two boys who were aged 9 and 12 at the time of the alleged offence in March. Mrs Cummins and her husband, John, have been the boys' legal guardians, together with the boys' biological mother, for six and a half years.

The boys have been diagnosed with attachment disorder and attention deficit disorder. Mrs Cummins said the younger boy felt jealous after the older one began to do well at college.

"For months he was picking up steak knives, scissors, paperclips, any sort of object that was sharp, and poking his brother with it," she said.

This culminated in the March incident when the younger boy stabbed his brother's leg with a pen that had been snapped so that it had a jagged edge.

Mrs Cummins said the older boy let out "a scream that made my blood run cold". She found out what had happened, and asked the older boy to sit next to the offender and jab the pen into his leg to show him what it felt like. "He didn't want to, so I picked it up and did the same to him [the offender]," she said.

Both boys told Judge McHardy that Mrs Cummins's action was "a stab". Mrs Cummins told him that she "took the pen in question, which was broken, and she brought it down on [the younger boy's] thigh area from a close vicinity to the thigh".

"There would seem to be no issue, as a result of this action on the defendant's part, there was bruising suffered to [the younger boy's] thigh," the judge said.

"It is quite clear that the evidence that the defendant has given me today falls within the category of her taking action which was intended to teach [the boy] a lesson."

But he found that Mrs Cummins' action was "reasonable" under a clause in the 2007 law that lets parents use force against a child for "preventing the child from engaging or continuing to engage in conduct that amounts to a criminal offence".

Former Children's Commissioner Dr Ian Hassall, who campaigned for the 2007 law, said the judgment appeared to breach the law's principle that force was not justified unless it was to "restrain" a child from committing a criminal offence, harming the child itself or another person or engaging in offensive or disruptive behaviour.

Police guidelines issued when the law was passed also state that force can only be used for "restraint".

"Reasonable force can only be used at the time that the intervention by the parent is required, i.e. force cannot be used after the event to punish or discipline the child," the guidelines said.

A police spokesman pointed to the guidelines yesterday but declined to comment on any specific case.

Mr McCoskrie said Prime Minister John Key seemed to have a policy of "out of sight, out of mind" for a growing list of cases where parents were "criminalised" under the 2007 law.

He said Tuesday's advertisements in the Herald, Dominion Post and the Press were paid for by donations raised within 48 hours of an email appeal to Family First's 6000 supporters.