There have been several articles in the newspapers over the last years talking to families where money has been spread unequally. Even when the child knew the reason for the inequality, they were still left with a devastating feeling of mattering less to their parent than their sibling/s.
Personally, I think parents should always discuss the terms of their will with their children and, generally share the money being left to children equally between them. We have certainly told our children the contents of our wills, who, other than they, will receive bequests, but that most of our estate will be shared equally between them. We were particularly careful about this as we have left some money to each of our DGC, and only one of our DC is a parent. The non-parent couldn't even see the problem, let alone our concern.