Within the Confessional
Canon Law specifically and absolutely forbids the priest from divulging anything sacramentally confessed (Can 983).
That said, and notwithstanding Canon 980, it would be possible for a priest to withhold absolution from a penitent until she has reported any crime to the authorities herself. The priest may legitimately have doubts about the penitent's penitence until she accepts the consequences of her actions.
I have been told that if a priest has plans to travel on a particular plane, and someone sacramentally confesses that that flight is to be hijacked and brought down, the priest must not alter his own behaviour and travel on a different plane. He must act as if he had heard nothing.
There is scope for conflict with civil law. In the United Kingdom, it is not normally an offence not to report criminal activity, but terrorism, money-laundering, treason and the failure to lawfully dispose of a body must be reported. A priest who hears such a confession is bound by civil law to report the crime and by canon law not to do so. Other countries may expand that list of applicable crimes.
Outside the Confessional
The requirement to maintain the secrecy of the sacramental confession does not bind anything confessed outside the sacrament of Reconciliation.
In the United Kingdom, it is not a criminal offence not to report or make an allegation of child abuse. Even if such a report is made, a priest could not give evidence such as "X told me that she had," because that's hearsay. Additional primary evidence would be required.
In England and Wales, the Catholic Safeguarding Advisory Service says
An adult who provides information should be encouraged to share that information with the statutory agencies, where appropriate.
They should be offered support by the Local Safeguarding Representative to do so.
If the adult or child providing information about another child request anonymity when the Safeguarding Officer makes the referral to Children's Social Care Services or the Police, those agencies will respect their wishes as far as possible. However, where investigations proceed to Court this may not be possible to maintain at all times.
An adult in an official or lay position of trust within the Church cannot expect to remain anonymous when a referral is made.
In the case of an offender confessing outside the Confessional, there is a duty to report, and to encourage the offender to tell the authorities herself.