I think the Parent should probably do the validation. So in the parent you might have a canBeParentOf(Child) method. This method would also be called at the top of your AddChild method--then the addChild method throws an exception if canBeParentOf fails, but canBeParentOf itself does not throw an exception.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.