I would start at the other end. Consult a lawyer. I'm not one, but I know that most states allow you a legal separation, which means you can set up temporary custody/visitation arrangements. This also has the advantage of making you not responsible for any debt your husband runs up or any damage he does to rental property during the time you're separated.
My state had a specific "separation agreement" that would allow you to separate for a year, and then decide -- either to get back together or, if you chose to divorce, the separation agreement would become your divorce agreement. But I know states differ on this.
I believe in being honest with the kids. I told mine about alcoholism, and that their father could get help but that he would have to choose to get help. And that as long as he didn't, we needed to make sure they were safe when they were with him.