A woman who poked a pen into the leg of a 9-year-old is the face of a Family First documentary on the anti-smacking law.

Henderson foster mother Lorriane Cummins decided to teach the boy a lesson after he stabbed his 13-year-old brother with a snapped ball point.

"I said to him 'You do to him what he did to you so he can see what it feels like'.

"He picked up the pen and made a half-hearted stab and I said 'no' and poked it into his leg."

The poke caused a bruise.

Cummins was arrested, but discharged without conviction four months later. She said the judge told her the incident was akin to "tapping a child on the hand".

The documentary My Mummy's A Criminal follows five parents criminalised by the law.

Family First national director Bob McCoskrie said politicians had ignored the public on the issue and hoped the documentary would help make the law an election debate.

"John Key and Phil Goff said if good parents were made criminals they'd change the law. Their integrity is now on the line. The evidence is they have been."