When parents separate children are always upset, even if they are not surprised. Parents need to understand this. Each child in the family is different and will cope in different ways with the emotions they are feeling, confusion, anger, sadness, depending on their age and understanding.
Research has shown that the manner in which parents deal with their separation/divorce can have a significant impact on how their children cope. It is the on-going conflict that negatively affects children. So the good news is there are things parents can do to help their children through the process. There are now a lot of books and websites that can help.
There are a few good books. “When Parents separate: helping your children cope” by John Sharry and others (Veritas) and “Living with separation & divorce” by Fiona McAuslan and Peter Nicholson.