I would definitely leave if I were in your shoes. I wonder what your financial situation is? Being a single mother can be tough, but there is nothing good or healthy in the relationship, based on your description. On some level you must love him still, but the co-dependency is not good for you, the kids, or his addiction frankly.
That's my two cents. I'm only being so frank because you asked. My heart really goes out to you. What a hard situation. It sounds like you've been very strong. I hope you have friends and family in your corner to help you and the kids, emotionally and financially.