Carry a tape recorder (or some device that can record your conversations) with you at all times and record every conversation you have with your wife. It may not be admissable in a court of law depending on your state, but it will help clear you of any abuse charges should she decide to accuse you of abuse.

And the next time she hits you, call the police and press charges.

You are planning on filing for divorce, aren't you? I would contact an attorney and see where you can go. Given what you have posted, you need to get custody of the kids. Do you have any witnesses to her abuse other than the children?