I'm sorry for your loss.
But you need to file a complaint against this dog. Not because it killed your cat, because its a dog and if not raised with cats it has no way of knowing it wasn't prey.But because it is a danger to everyone in the neighborhood
If this pit bull is chained and is...
I would just explain that your children need closure and you would like to hold a ceremony for them . And we could say a prayer. We are upset but we know nobody is at fault. A letter to this affect might work better than in person.