you can add a handleNotification method to the parent editpart this will inform you about all changes in its content including things like old and new value. You can also attach a listener to your editing domain, or to the model which would be a more gmf independent approach.
Thnaks Ralgh for your reply,
Can u help me more and tell me what code should i add to my project ?
I will give u the EditPart of the parent(EchelleSpecifique.EditPart) and of the child(Composant.EditPar)
And a lot for your Help my bro