In my state, there was no such thing as a separation agreement, really. If you were separated and did not want to divorce, you could make a support agreement using the same state calculators as you do for divorce. I'm not sure why one would not want to divorce, though. I guess this is because I'm Jewish and I knew that I couldn't have a religious divorce without having a civil one. (Also, in Judaism, there is divorce. Catholics have to apply for annulment. By the way, if you are Catholic, and your husband is a substance abuser who blames you for-- let me just say, I think from what I've read about annulments that the church would give you an annulment.)
I don't know what the situation is in Illinois. I checked using Google and found this: http://www.illinoisdivorce.com/famil...separation.php
It sounds like the only difference between legal separation and divorce is that you can't remarry if you have a legal separation, and that Illinois divorce lawyers use this tactic only rarely.
What is it that you were hoping would happen in a separation that is different from a divorce? It sounds like what you want is a divorce. You may never want to marry again, I know. But perhaps you will want to. Maybe you will want to like or love someone new. I found it really hard to have my heart open to anyone while I was still married, even with my ex out of my house.
If what you are looking for is a trial separation, that's something you have to work out with your spouse himself.
Your reasons here are good. There is nothing at all wrong with saying that you don't want to be married to someone who abuses alcohol and other drugs. There are so many strong parenting reasons to get out of this. It's going to hurt emotionally anyway, but it sounds like the right thing. You don't trust him to take care of the kids. The kids themselves are scared. You know he can do better, but the fact is, you don't know that he'll ever do better with you.