Nothing but their children’s deaths could stop the parents’ anger.
The first line is about how the parents hatred never ends. It just keeps continuing. The second line translates to saying that the death of the children is the only thing that can remove the anger they have for each other. This is a shame because the feuding between the families can be seen as one of the causes of Romeo and Juliet's deaths. If the families had reconciled earlier, Romeo, Juliet and Tybalt my all still be alive.
