The landmark six-hour session before the UN's child rights watchdog in the Swiss city came as Pope Francis said Catholics should feel "shame", in an apparent reference to the scandals that have rocked the Church.
Francis, who has vowed zero tolerance of abuse, last month created a special commission to investigate sex crimes, enforce prevention and care for victims.
Monsignor Charles Scicluna, the Vatican's former top prosecutor, said the Church understood what it had to do.
"The Holy See gets it, that certain things have to be done differently," he told the UN Committee on the Rights of the Child.
"It's not words, it has to be commitment on the ground, on the level of the local churches," he added.
But the UN committee questioned the Vatican's resolve.
"I have my doubts about the change in attitude," committee representative Sara Oviedo told the Vatican delegation.
"You need to get down to business. We need to see concrete actions."
The Catholic Church has been shaken by a decade of scandals involving child abuse by priests and lay officials, from Ireland to the United States and Australia.
Pressed for details of the new commission, the Vatican's UN ambassador Monsignor Silvano Tomasi said its ground rules and membership were still being established.
Oviedo also demanded to know what the Vatican was doing in the case of Archbishop Jozef Wesolowski, a papal envoy from Poland recalled to Rome from the Dominican Republic amid claims of abuse.
Without naming the envoy, the Vatican said it was investigating the case.
But victims' groups question its commitment to turn rhetoric into reality.
"These are just yet more empty words," Polish campaigner Marek Lisinki told AFP in Geneva.
"They keep telling us what they're doing, but there's a lack of concrete answers."
Abuse has often been covered up by priests' superiors, who typically transferred offenders to new parishes, rather than turn them over to police.
Scicluna said that was no longer the case.
"It is a no-go simply to move people from one diocese to another," he said.
"There is no place in the priesthood for anyone who would harm children and the young," Scicluna said.
"It is not a policy of the Holy See to encourage cover-ups," he said.
"Our guideline has always been that domestic law of the countries where the churches operate needs to be followed."
'It's the children who matter"
Like other signatories of the 1989 UN Convention on the Rights of the Child, the Vatican agrees to submit regular reports on its respect for the rules, and to be scrutinised by an 18-member panel.
Thursday's session marked the Vatican's second appearance before the committee.
The first was in 1995, before the abuse cases burst into the spotlight.
The committee's recommendations are due to be issued on February 5th.
They are non-binding but carry moral weight.
"It's important that the meeting we have had today bears some fruit," said committee member Hiranthi Wijemanne.
"At the end of the day, it's the children who matter."
Veronica Yates, director of British-based Child Rights International Network, underlined the importance of Thursday's hearing.
"The message is clear from the UN. You're not complying. You're not willing to respond to your obligations, he told AFP.
"You're denying and diverting. It's the first time that an international body that promotes human rights has said this,"
Benedict XVI, pontiff from 2005 to 2013, was the first pope to apologize to abuse victims and call for zero tolerance.
Pope Francis alluded to abuse in a homily on Thursday.
"Do we feel shame? There are so many scandals that I do not want to name them individually but everyone knows about them!" he said.
Campaigners said such "lofty words" were undermined by the fact that Francis met Thursday with retired Los Angeles Cardinal Roger Mahony, stripped of his duties last year over cover ups.
Barbara Blaine, head of the US-based global network Survivors' Network of those Abused by Priests, said there were two benchmarks for the Vatican.
"Number one is, are they turning the evidence of sex crimes over to the police? And number two, are they punishing any of the bishops who have enabled and covered up for the sexual predators?"
"That would give me hope that children would be protected in our church. But they fail to do either one," she told reporters.
Tomasi said the Vatican was serious about fighting abuse, calling it a "wound that hurts the Church as a community of faith".
But he argued that its powers were limited given that it was legally responsible only for implementing the UN children's rights convention on the territory of Vatican City.
Committee head Kirsten Sandberg shot that down.
"All these offenders are employed by the Catholic Church," she said.
The Vatican says it receives around 600 claims against abusive priests every year, many dating back to the 1960s, 1970s and 1980s.
Victims' groups say the issue is far from settled with cases of ongoing abuse emerging regularly and the tally potentially in the hundreds of thousands.