This is will be a very hard one to forgive. It will take the grace of God for someone to let go of such hurt of losing a child when the attacker comes forward asking for forgiveness.
In such a situation, I might have to forgive after justice has taken it's full course first. There ought to be punishment for certain attrocities before atoning for mercy.
It sure wouldn't be easy. I will have to let the scripture take it course, as it said forgive others. If our Heavenly Father can forgive us for the worst sins, it's good to do likewise to our fellow no matter how hard it might be. It's not good to take such offense to the grave.