I'm so sorry to hear what happened to your Chi Pongo. That must have been absolutely devastating.
It really sounds like Copper ended up in the right home. I wonder if he had anxiety issues around children. Some dogs feel absolutely uncomfortable around children.
I think it was for the best that you found a different home for him. Aggression is definitely not acceptable with children, especially young children. An aggressive dog can do a huge amount of damage to a child, even dogs smaller than a beagle.
Sometimes, dogs end up not being the right fit for their owners. It's not necessarily the owner's fault. In some situations, they are better off in a home that is a better fit for them.