OK, well, on the good side, there's not a whole lot they will have to testify about. Maybe their observations, if any, about dad's condition when they got in the car, maybe whether they were scared. How he was driving. That's about all I can think of they could possibly be questioned about. Their testimony would probably be not more than a few minutes on the stand.
Hopefully it won't come to that, but if it does, handling it the right way can make this a teachable moment. Nobody is above the law. If you break the law and put people in danger you will be held responsible. And doing the right thing, like testifying when you know something about what happened, is everyone's job. Good citizenship.
I know that from where you sit this is a family matter, but remember, he could have not only hurt your kids driving in that condition, he could have hurt other people. He needs to be dealt with, and the system is trying to make that happen.
Hugs,