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"If someone was to see the abuse ... they would have an obligation to do something," Bridges said.
"There would though be a reasonable excuse defence around, for example, being fearful for themselves."
The new offence would be different to the proposal in National MP Chris Penk's member's bill, which would enable a judge to draw a negative inference from a defendant who refused to give evidence.
But Little was dismissive of Bridges' proposal.
"I want to know whether this will add anything or whether it's just populist talk."
Little said compelling people to talk to police was a "fraught issue", as it could end up harming innocent people.
The right to silence was a fundamental human right and should not be changed without rigorous public debate, he said.
"What powers do you want the police to have, bearing in mind that those powers will be used in any other occasion, including in relation to people who might not have harmed anyone but were in close proximity?"
Little has asked Justice Ministry officials to look at the British system of police cautions at the time of arrest.
"This is along the lines of, 'You have the right to remain silent but you may harm your defence if you fail to say anything you later rely on in court.'
"I think this approach offers a better balance between the right to silence and the ability to be properly held to account in court."
Prime Minister Jacinda Ardern has urged the family in the case of the 4-year-old to come forward.
She supported Little looking at police cautions.
"I don't think anyone is comfortable [with] what's happened in this case, when no one is being help responsible for the harm to a vulnerable child, so we're looking into it."
She said changes under the previous National-led government did not appear to have worked.
National had introduced a new offence in 2011 of failing to protect a child or vulnerable adult; so far 34 charges have been laid against 25 people.