If you didn't have kids I would say to testify but if they retaliate in anyway your kids are going to be there to see or hear.
If you go over there she could go off at you and be violent or she could decide to talk to you and befriend you. The latter is not as good as it sounds because next time he starts on her she might run to your house for help and he would follow and suddenly the whole fight is going on in your home in front of you and the kids.
I would just call the cops every time. It's horrible to let it happen but you need to put your own family first