If you are leaving your marriage because your spouse has a drug and/or alcohol problem and will not seek help, you have to assess whether your spouse could present a danger to you and your children if you leave.
The best predictor of violence is past violence, but what some drugs like methamphetamines, Crack, or cocaine, a past that has no violence may not matter. It is important to seek the help of an addiction specialist and a family law attorney or center. There are services for low income families across the country for issues like this.
For instance, the Chabad Centers are in every major city and are run by the Jewish Community, but service anyone in need. They are devoted to helping people who are dealing with issues of alcohol and drug abuse. They typically have connections to attorneys that work on a pro-bono (for free) basis that can give you advice as to how to proceed. This, as well as domestic violence, is two areas in which you do not tell the children together about your separation.
You may, depending on the level of alcohol or drug abuse, need a restraining order and a plan to get yourself and your children out of the home safely (or the other party out of the home).