I've always felt that a parent should be honest. WW doesn't want that guilt, but it is her actions that are doing this to the family, too. I would say that telling them that "mommy isn't happy and doesn't want daddy anymore" is the truth. Make sure they feel loved and make sure that you tell then that you are not done, yet, too. They need to see you strong right now.
WW wants you to say that it was a mutual decision because then she doesn't have to face the kids with the truth.
