That's about all I would say. And perhaps, I love you and I'm here for you no matter what.
They do not need to know your heart is breaking nor that they are about to "go through" something horrible.
Don't you know that they will see and feel your energy and know the gravity of what is happening? Don't compound it or project. Be strong enough to watch them take in the information and be there for them.
This is not about you making sure they know it is not your fault or that you "get it." In reality, you don't know how they are going to process this. Let them show you and share with you.