It is my opinion that the exact message you deliver will depend on your particular children, their maturity levels, and their attitudes. I personally do not believe it is necessary to talk a lot about your AH's disease, unless the older kids need certain information in order to protect themselves when alone with him. However, I believe there are three important underlying messages to give when informing the kids:
1. You are loved. I love you. Your dad loves you. Your grandparents love you. Some things in our family are going to be changing, but our love for you has not changed and will not change.
2. This is a grown-up situation, and the grown-ups will handle it. Dad and I are trying to work through the best way to handle the changes that are coming. If you have any questions, or are worried about something, you can ALWAYS ask me and we'll talk things through. But your only "job" is to keep being kids.
3. (and, if your children have ever expressed fear of their father) You are safe.
It's a delicate balance to strike with older kids: giving them enough information so they are aware enough to keep themselves safe, etc., without burdening them with so many details that it creates a lot of unnecessary anxiety. Maybe consider Alateen for your older kids?