Children witness domestic violence in many different forms. For example, they may get caught in the middle (trying to stop mum/dad being abused) .
They may hear verbal/physical abuse from a different room,they may even see mum/dad with a "panda eye" or "fat lip"
my point is like Margaret says the children that witness domestic violence are being (emotionally) abused themselves.
Here is a interesting read for those who wish to know more.
Domestic violence - its effects on children: information for parents, carers and anyone who works with young people