This question is not entirely hypothetical. This situation recently occurred in a Chicago suburb.
A woman recently murdered her kids. She went to confession earlier that afternoon.
While the act seems premeditated, we don't know if she mentioned it to the priest. For the sake of the discussion, let's say she did.
What options are open to the priest? Can he contact the police and tell them that he believes that the children are in danger without telling them how he knows?
Is this a true sacramental confession? It's my understanding that you can't confess a sin that you intend to commit and receive absolution. If that's the case, does the seal of confession apply? She's confessed to a plan, not a sin.
In this particular case the police want the priest to tell them what he knows but he has not. Is he in the right in this case? Obviously he can't say anything about other things she may have confessed but if she told him what she was going to do wouldn't the saving of innocent life outweigh the seal if it did apply? It's one thing for a priest to go to a martyr's death but how can he make that decision for someone else?