What I had to do was to say that my children would not get into a car with him if he had been drinking OR WAS ACTING ERRATICALLY IN ANY WAY. That took care of the whole denying the drinking thing. I also have it in my divorce decree I can test him at any time he has our children.
You need to take your kids to counseling so they can build up the strength to address their father. If they are scared of him and won't tell you he has been drinking, you are done. I took my kids to counseling for someone else to get through to all of them that you cannot get into a car with ANYONE if they are drinking or acting strange, under any circumstances. Early on, we had to use this too. He drank and they refused to get in the car. They called me immediately on cell phones I supply to them and I came and got them.
Get a counselor who specializes in helping families with addiction. Also, speak to your attorney.