It's sad to say that there was absolutely nothing you could do to protect your children. It's really no one's fault except for the people who abuse them. Kids are so innocent and ignorant to the possibility that someone out there has the intention of hurting them. Perhaps it was made worse because they didn't tell you. Whatever it was, you had no control over their abuse. I don't know what else to say, some people are just ****s.
Cheers,
Peter